• * * i *

ISSN 1018-5593

European Commission

technical steel research

Reduction of iron ores

The production of stainless steel through smelting reduction of chrome ores

using coal and oxygen

STEEL RESEARCH

European Commission

technical steel research Reduction of iron ores

The production of stainless steel through smelting reduction of chrome ores

using coal and oxygen

C. Treadgold British Steel, Teesside Technology Centre

Eston Road, PO Box 11 Grangetown

Middlesbrough TS6 6UB United Kingdom

Contract No 7210-AA/811

1 July 1988 to 30 June 1990

Final report

Directorate-General Science, Research and Development

1997 EUR 13956 EN

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Luxembourg: Office for Official Publications of the European Communities, 1997

ISBN 92-828-1483-1

© European Communities, 1997 Reproduction is authorised provided the source is acknowledged.

Printed in Luxembourg

THE PRODUCTION OF STAINLESS STEEL THROUGH SMELTING REDUCTION OF CHROME ORES USING COAL AND OXYGEN

British Steel Technical

ECSC Agreement No. 7210.AA/811

Final Summary Report

The primary objective of this research programme was to develop a coal-based smelting reduction process to produce a liquid chrome-iron suitable for subsequent refining to stainless steel. If successful, such a process would enable manufacture of stainless steel to take advantage of chromium units, in the form of chromite ore concentrates, which would be cheaper than the conventional alloys of chrome and iron produced by a submerged arc electric smelting step. Further cost reductions and energy savings would be achieved by utilising the chrome-iron product in its liquid state, thus removing a second electric powered melting step from the process route.

An initial theoretical evaluation of the proposed coal-based smelting step identified the key process parameters of slag composition, metal carbon concentration and process temperature and a pilot plant scale experimental programme was carried out to evaluate process operation at ranges of those key parameters. In addition the experimental programme studied the effects of composition of the ore and coal, the method of feeding of the ore and coal, the physical properties and behaviour of the slag and the degree of mixing both in the slag phase and between the metal and slag.

Thirty experiments were carried out using two pilot plants, each of nominal 3 tonnes capacity, located at British Steel Technical's Teesside Laboratories. The plants shared common control and instrumentation facilities, enabling the same process measurements to be taken on either plant. One was a purpose-built pilot plant designed to enable studies of continuously operating smelting reduction processes to be carried out, and the other was a 3 tonne scale BOS-type converter. The plants differed in their modes of operation with respect to gas extraction, and in the geometry of the vessels. These differences enabled complementary studies to be carried out of gas generation and composition on one plant, and of physical aspects of slag behaviour such as foaming, on the other.

The experimental programme identified slag foaming as a potential process difficulty. This problem was overcome by a combination of techniques designed to modify the factors that govern the generation and stabilisation of slag foams. Small quantities of surface acting materials, notably sulphur, were used to modify surface tension effects. Slag silica contents were minimised in order to reduce slag bulk, and basicities of between 2.0 and 2.5 were used to reduce slag viscosity. High velocity injection of the ore particles into the oxygen jet cavity region was practised in order to minimise the presence of solid chromite particles in the slag phase. These actions were successful in controlling slag foaming, although it was not possible to isolate the effects of each action in the results of this experimental programme. Slag foaming was identified as the limit to productivity for a converter shaped vessel, and the specific throughputs used in this work were close to that limit.

High bath carbon concentrations, of 5% or more, and minimum process temperatures of 1600°C were found to be necessary in order to achieve acceptable chromium yields of 90% and greater.

High yields were achieved over the full range of chromium concentrations required for the production of a stainless steel refining stock. A clear correlation between bath carbon concentration and chromium yield was found. A further correlation between process temperature and yield was discovered, but no clear effect of slag composition on reduction efficiency was found. These observations, together with microscopic examination of slag samples taken from the experiments have been used to deduce the likely reaction mechanisms. It is postulated that the reduction of the chromite particles proceeded by a diffusion controlled "shrinking core" mechanism, in which diffusion of the reductants into and the various ionic species out from the chromite particles took place. Carbon dissolved in the metal was identified as the major reductant, with the principal reaction site at the metal particle interface. This mechanism was aided by the high velocity injection technique referred to above.

An examination of the generation and composition of process gases was undertaken in the experimental programme. This showed that a gas of consistent quality and flow rate was produced, with potential uses as a fuel gas or in generation of electricity. The gas composition was dependent upon the coal type used in the experiments, and had a typical calorific value of 9.2 MJ/Nm3. Operation of the smelting reduction step in closed hood mode, would enable collection of this gas and the recovery of its calorific value.

The experimental results were compared with the predictions of a heat and mass balance mathematical model of the process in order to establish the theoretical processing time and raw materials consumptions to produce the chrome-iron product at a commercial scale of operation.

An examination of the decarburisation and refining steps required to convert the chrome-iron product to stainless steel was carried out. The high carbon content (5% or more) of the chrome-iron would extend blowing times in conventional AOD refining to unacceptable levels and would also impose excessive heat loads on the uncooled hood of a typical AOD plant. These problems could be overcome by adoption of a top-blown variant of the AOD process up to the end of the first stage of decarburisation, typically 0.4% C. A vessel equipped with a BOS style of hood would be required, and if this were operated in the closed hood mode additional energy savings could be made by collection of the process gases in the first stage of refining.

Control of phosphorus is essential in stainless steelmaking, and there are no techniques currently available, at the commercial scale of operation, for dephosphorisation of stainless steel. Control must, therefore, be by careful choice of the raw material inputs. The results of the experimental programme identified the initial hot metal charge as the source of over 90% of the input phosphorus. This indicates that known hot metal dephosphorisation techniques would be effective in controlling the phosphorus content of the final stainless steel product. Careful selection of the coal type used would also be beneficial in this respect. Control of sulphur remains as a technical uncertainty in the process.

These results and arguments have been used to identify the key process elements required to produce stainless steel by this proposed route. Two alternative process options have been identified. One would utilise the electric arc furnace to melt a combination of plain and stainless steel scrap, followed by the smelting reduction and decarburisation steps. This option would allow maximum utilisation of the chromium and nickel content of stainless steel scrap.

The second option would be to operate the process in an integrated iron and steelworks, in which the blast furnace hot metal would be desiliconised and dephosphorised prior to the smelting reduction step. This could be carried out either in a purpose-built vessel, or in a suitably modified BOS converter. The converter would require facilities for coal and ore preparation, pneumatic injection through a top lance, basal stirring by inert gas and process gas collection. Refining of the

chrome-iron could either proceed in an adjacent converter, or possibly in the same vessel after deslagging. Capital costs for either process option would be strongly site specific and would depend upon the facilities already present in the steel plant. The evaluation of those capital costs was, therefore, beyond the scope of this study.

A preliminary assessment of the raw material costs of the proposed process indicates significant potential savings when compared with the conventional submerged arc-electric melting-AOD route. It is recommended that an in-depth economic evaluation be undertaken to confirm these indicated savings.

CONTENTS Page

1. INTRODUCTION 21

2. THEORETICAL CONSIDERATIONS 22

2.1 Reaction Mechanisms 22

2.2 Thermodynamic Considerations 23

3. EXPERIMENTAL PROGRAMME 24

4. EXPERIMENTAL RESULTS 25

4.1 Slag Foaming Behaviour 26 4.2 The Effect Of Metal Carbon Content On The Reduction Of Chromite 27 4.3 The Effect Of Process Temperature On The Reduction Of Chromite 27 4.4 The Effect Of Slag Basicity On The Reduction Of Chromite 27 4.5 Microscopic Examination Of Slag Samples 28. 4.6 Gas Generation 28

5. PROCESS CHARGE BALANCE 29

6. THE PRODUCTION OF STAINLESS STEEL 30

6.1 Decarburisation 30. 6.2 Control Of Phosphorus And Sulphur 31 7. PRELIMINARY ECONOMIC EVALUATION 31

8. CONCLUSIONS 32

9. REFERENCES 34

TABLES 35

FIGURES 44.

APPENDIX 64

LIST OF TABLES

1. Experimental Parameters 2. Composition of Raw Materials 3. Raw Material Particle Size Analysis 4. Summary of Materials Inputs and Outputs 5. Chromium Mass Balances 6. Specific Raw Material Consumptions 7. Materials Balance to Produce 100 tonnes of Chrome Iron 8. Raw Material Consumptions in Stage One Decarburisation 9. Raw Materials Costs Smelting Reduction Route

LIST OF FIGURES

1. Liquidus Surface of the CaO-MgO-Si02-Al203 System at the 20% A1203 Plane showing the Aim Slag Composition

2. Liquidus Surface Iron-Chrome-Carbon System 3. Main Features of Pilot Plant 4(a) C.S.M. Heat 103-Process Summary 4(b) C.S.M. Heat 103 - Process Summary 5(a) C.S.M. Heat 104 - Process Summary 5(b) C.S.M. Heat 104 - Process Summary 6(a) C.S.M. Heat 105 - Process Summary 6(b) C.S.M. Heat 105 - Process Summary 7. Mean Bath Carbon vs Chrome Recovery - C.S.M. Mass Balances 8(a) C.S.M. Heat 102 - Process Summary 8(b) C.S.M. Heat 102 - Process Summary 9. Mean Bath Temperature vs Chrome Recovery - C.S.M. Mass Balances 10. Slag Basicity vs Chrome Recovery 11. Micrograph of a Reacting Grain from Experiment 105 12. Element Distribution of a Reacting Grain from Experiment 105 13. Duct Gas Analysis from Heat C.S.M. 81 14. C.S.M. Heat 81 Waste Gas Volume Flow Rate 15. Smelting Reduction of Chrome Ores - Comparison between Experimental Results and

Calculated Chromium Levels 16. Potential Process Routes

90/0153

LA PRODUCTION DE L'ACIER IMOXYDABLE PAR LA REDUCTION PAR FUSION DE CHROMITES, UTILISANT LE CHARBON ET L'OXYGENE.

British Steel Technical

Agrement CECA No.7210-.-AA/811

Sommaire du rapport final

L'objectif primordial de ce programme de recherche fut le developpement d'un procede de reduction par fusion a base de charbon afin de produire use chromite-liquidescapable d'etre par la suite raffinee en acier inoxydable. Si reussi, ce procede permettrait aux fabricants d'acier inoxydable de beneficier des unites de chrome sous la forme de concentres de chromite, qui seraient moins chers que les alliages de chrome et de fer conventionnels produits par la fusion electrique sous flux. D'autres epargnes de cout et d'energie seraient realisees en utilisant les chromites a l'etat liquide, ainsi eliminant une deuxieme etape de fusion electrique au programme du procede.

Une evaluation initiale de l'étape de fusion a base de charbon proposee identifia les parametres-cles du procede de la composition des scories, la concentration metal carbone et la temperature du procede, et un programme experimental a l'echelle pilote fut effectue pour evaluer le fonctionnemet du procede sur l'etendue de ces parametres-cles. De plus, le programme experimental etudia les effets de la compo­sition des chromites et du charbon, la methode d'alimentation des chromites et du charbon, les proprietes physiques et le comportement des scories et le niveau de malaxage autant a la phase scories qu'entre le metal et les scories.

Trente essais furent realises en utilisant deux installations pilotes, chacune d'une capacite nominale de 3 tonnes, situees aux laboratoires de la British Steel a Teesside. Les installations partagerent les memes systemes de commande et d'ins­trumentation, permettant ainsi de prelever les memes mesures de traitement sur chaque installation. L'une de celles-ci fut specialement construite pour permettre d'etudier continuellement les procedes de reduction par fusion, et l'autre etait un convertisseur type BOS de 3 tonnes. Les installations etaient differentes quant a leur mode d'operation de 1'extraction du gaz et de la geometrie des appareils. Ces differences permirent d'effectuer des etudes complementaires de la generation et composition du gaz sur une installation, et des aspects physiques du comporte­ment des scories, tel que l'ecumage, sur 1'autre.

Le programme experimental identifia l'ecumage des scories comme etant potentiel-lement un probleme du procede. Ce probleme fut resout par la combinaison de methodes destinees a modofier les facteurs qui gouvernent la generation et la stabilisation des ecumes de scories. De faibles quantites de substances tensio-actives, en parti-culier le soufre, furent utilisees pour modifier les effets tensio-actifs. Les teneurs en silice de scories furent minimisees de façon a reduire le pourcentage de scories, et des basicites entre 2,0 et 2,5 furent utilisees pour reduire la viscosite des scories. L'injection a haute vitesse de paticules de chromite dans la cavite du jet d'oxygene fut utilisee pour minimiser la presence de particules de chromite solide a la phase des scories. Ces interventions reussirent a controler l'ecumage des scories, bien qu'il ne fut pas possible d'isoler les effets de chaque

intervention dans les resultats de ce programme experimental. L'ecumage des scories fut identifie en tant que limite de la productivity d'un appareil type convertisseur et les rendements specifiques utilises dans ce programme furent proches de cette limite.

De hautes concentrations de carbone de 5% ou plus dans le bain, ainsi qu'une temperature minimale d'environ 1600 C furent exigees afin de realiser des niveaux de production de chrome acceptables de 90% et plus.

De hauts rendements furent realises sur toute la gamme de concentrations de chrome necessities pour la production d'ebauches d'acier inoxydable a raffiner. Une cor­relation marquee fut temoignee entre la concentration de carbone dans le bain et le rendement de chrome. Une correlation supplementaire entre la temperature et le rendement du procede fut decouverte, mais aucun effet marque n'en resulta entre la composition des scories et l'efficacite de la reduction. Ces observations, ainsi que l'examen microscopique des echantillons de scories preleves des essais furent utilises pour deduire les mecanismes de reaction probables. II est postule que la reduction des particules de chromite fut effectuee par un mecanisme de "noyau retre-cissant" control! par diffusion, dans lequel eut lieu la diffusion des reducteurs dans les particules de chromite et les differentes especes ioniques hors de celle-ci. Le carbone dissout dans le metal fut identifie en tant que reducteur principal, avec la zone de reaction principale a 1'interface des prticules metalliques. Ce mecanisme fut aide par l1injection a haute vitesse mentionnee ci-dessus.

L'examen de la generation et la generation et la composition des gaz du procede fut effectue dans le programme experimental. Ceci indiqua qu'un gaz d'une qualite et d'un debit constants, potentiellement utilisable en tant que gaz de chauffage ou de generation d'electricite fut produit. La composition du gaz fut dependante du type de charbon utilise dans les essais et avait une valeur calorifique typique de 9,2 MJ/Nm . Le procede de reduction par fusion en hotte fermee, permettrait de collecter ce gaz et de recuperer sa valeur calorifique. Les resultats experimentaux furent compares aux predictions d'un modele mathematique d'equilibre de chaleur et de masse du procede de fa?on a etablir le: temps theorique du procede et les consom-mations de matieres premieres pour produire la chromite a l'echelle commerciale.

Un examen de la decarburation et des etapes de raffinage exiges pour convertir la chromite en acier inoxydable fut effectue. La haute teneur en carbone (5% ou plus) de la chromite aurait pour effet d'augmenter les temps de soufflage du raffinage AOD conventionnel jusqu'a un niveau inacceptable, et imposerait egalement des charges thermiques excessives sur la hotte non-refroidie d'une installation AOD typique. Ces problemes pourraient etre resouts par 1'adoption d'une variante soufflee en tete du procede AOD jusqu'a la fin de la premiere etape de decarburation, typique-ment 0,4% C. Un appareil equipe d'une hotte du type BOS serait necessaire, et si celui-ci fonctionnait avec une hotte fermee, des epargnes d'energie seraient pos­sibles en collectant les gaz du procede a la premiere etape du raffinage.

Le controle du phosphore est indispensable a la production de 1'acier inoxydable et, a l'heure actuelle, il n'existe aucune methode a l'echelle commerciale pour dephosphoriser 1'acier inoxydable. Le controle doit etre effectue par le choix soucieux des matieres premieres utilisees. Les resultats du programme experimental identifierent la charge de metal liquide initiale comme etant plus de 90% de la source de phosphore. Ceci indique que les methodes connues de dephosphorisation du metal liquide seraient efficaces pour le controle de la teneur en phosphore du produit final en acier inoxydable. Le choix soucieux du type de charbon utilise serait egalement avantageux a cet effet. Le controle du soufre demeure une incer­titude technique dans ce procede.

10

Ces resultats et arguments ont ete utilises pour identifier les elements-cles du procede exiges dans la production de 1'acier inoxydable par cette methode proposee. Deux options alternatives du procede ont ete identifiees. L'une utili-serait le four a arc electrique pour fondre la combinaison des ferrailles d'acier ordinaire et inoxydable, suivie des etapes de reduction par fusion et de decarbu-ration. Cette option permettrait d'utiliser les teneurs maximales en chrome et en nickel de la ferraille d'acier inoxydable.

La deuxieme option serait d'effectuer le procede dans une usine siderurgique integree, dans laquelle le metal liquide du haut-fourneau serait desiliconise et dephosphorise avant l'etape de reduction par fusion. Ceci pourrait etre effec-tue soit dans un appareil specialement construit, soit dans un convertisseur BOS modifie d'une fa^on appropriee. Le convertisseur exigerait des prestations de preparation du charbon et de la chromite, injection par lance superieure, agita­tion basale par gaz inert et collection du gaz du procede. Le raffinage de la chromite pourrait soit etre effectue dans un convertisseur adjacent, soit, even-tuellement, dans le meme appareil apres le decrassage. Les frais capitaux de chaque option de procede dependraient principalement des conditions sur place et des prestations existantes dans l'usine siderurgique. L'evaluation de ces frais capitaux etait done au-dela des limites de cette etude.

Une evaluation preliminaire des couts de matieres premieres du procede propose temoigne 1'importance potentielle des epargnes eventuelles par rapport a la methode conventionnelle de la fusion electrique sous flux AOD. II est recommande d'effec­tuer une evaluation economique approfondie afin de confirmer les epargnes precitees.

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TABLE PES MATIERES page 1. AVANT-PROPOS 2 1

2. CONSIDÉRATIONS THÉORIQUES 22 2.1 Mecanismes de reaction 22 2.2 Considerations thermodynamiques 23 3. PROGRAMME EXPERIMENTAL 4. RESULTATS EXPERIMENTAL

6.1 Decarburation

24

25

4.1 Comportement de l'écumage des scories 26 4.2 Effet de la teneur en carbone du métal sur la réduction de la chromite 27 4.3 Effet de la temperature du procede sur la reduction de la chromite 27 4.4 Effet de la basicite des scories sur la reduction de la chromite 27 4.5 Examen microscopique des echantillons de scories 28 4.6 Generation du gaz 28 5. EQUILIBRE DE LA CHARGE DU PROCEDE 29 6. PRODUCTION D'ACIER INOXYDABLE 30

30 6.2 Controle du phosphore et du soufre 31

7. EVALUATION ECONOMIQUE PRELIMINAIRE 3 1

8. CONCLUSIONS 32 9. REFERENCES 34

TABLES 35 FIGURES 44 ANNEXE: 6 4

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LISTE PES TABLES

1. Parametres experimentaux 2. Composition des matieres premieres 3. Analyse de la taille des particules de matieres premieres 4. Sommaire des entrees et sorties de materiaux 5. Equilibres des masses de chrome 6. Matieres premieres specifiques consommees 7. Equilibre des materiaux pour la production de 100 tonnes de chromite 8. Matieres premieres consommees a la premiere etape de decarburation 9. Couts des matieres premieres de la methode de reduction par fusion

LISTE DES FIGURES

1. Superficie liquidus du systeme CaO-MgO-Si0„-Al„0„ aun plan de 20% A1„0„ indiquant la composition des scories prevue.

2. Superficie liquidus du systeme fer-chrome-carbone 3. Principales caracteristiques de 1'installation pilote 4(a) Chaude 103 C.S.M. - Sommaire du procede 4(b) Chaude 103 C.S.M. - Sommaire du procede 5(a) Chaude 104 C.S.M. - Sommaire du procede 5(b) Chaude 104 C.S.M. - Sommaire du procede 6(a) Chaude 105 C.S.M. - Sommaire du procede 7. Moyenne de carbone dans le bain v recuperation du chrome -

Equilibres des masses C.S.M. 8(a) Chaude 102 C.S.M. - Sommaire du procede 8(b) Chaude 102 C.S.M. Sommaire du procede 9. Moyenne de la temperature du bain v recuperation du chrome -

Equilibres des masses C.S.M. 10. Basicite des scories v recuperation du chrome 11. Micrographique d'un grain reagissant depuis l'essai 105 12. Distribution des elements d'un grain reagissant depuis l'essai 105 13. Analyse du gaz dans la conduite depuis la chaude 81 C.S.M. 14. Debit volumique du gaz pauvre de la chaude 81 C.S.M. 15. Reduction par fusion des chromites - Comparaison entre les resultats

experimentaux et les niveaux de chrome calcules. 16. Methodes potentielles du procede

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Produktion rostfreier Stahle durch Schmelzreduktion der Chromerze mit Kohle und Sauerstoff British Steel Technical EGKS Vertrag Nr. 7210.AA/811 Zusammenfassender SchluBbericht Das Hauptziel dieses Forschungsprogramms ist die Entwicklung eines kohlebasierten Schmelzreduktionsprozesses gewesen, urn ein flüssiges Chromeisen zu erzeugen, das fur anschlieBende Ver-feinerung zu rostfreiem Stahl geeignet ist. Wenn dieser ProzeB erfolgreich ist, dann konnten die Produzenten rostfreier Stahle die Vorteile aus Chromquellen in Form von Chromeisenerzkonzen-traten Ziehen, die billiger als die herkommlichen Legierungen aus Chrom und Eisen sind, die mit UP-LichtbogenschweiBen produziert werden. Wenn das Chromeisenprodukt im flüssigen Zustand verwertet wird, sind eine weitere Herabsetzung der Kosten und Energieein-sparungen moglich, weil dadurch ein zweites Elektro-Schmelzen in der ProzeBroute eliminiert wird. Wahrend einer ersten theoretischen Bewertung des vorgeschlagenen kohlebasierten Schmelzprozesses hat man die wichtigsten ProzeB-parameter wie die Schlackenzusammensetzung, die Metallkohlen-stoffkonzentration und die ProzeBtemperatur identifiziert, und deshalb ist ein experimentelles Programm im Versuchsmaßstab durchgefuhrt worden, urn die ProzeBfunktion fur verschiedene Werte dieser Parameter bewerten zu konnen. AuBerdem hat man wahrend dieses Programms auch die Auswirkungen der Erz- und Kohle-zusammensetzung, die Methode der Erz- und Kohlebeschickung, die physikalischen Eigenschaften der Schlacke und das Ausmaß des Mischens beider in der Schlackenphase und zwischen dem Metall und der Schlacke untersucht. Man hat dreiBig Experimente in zwei Versuchsanlagen durchgefuhrt, von denen jede eine Kapazitat von 3 t hat, und sie befinden sich in den British Steel Technical Laboratorien in Teesside. Beide Anlagen hatten eine gemeinsame Steuerung und Instrumentierung, so daB man die selben ProzeBmessungen in der einen oder anderen Anlage machen konnte. Die eine Versuchsanlage ist dem Zweck entsprechend gebaut worden, so daB Untersuchung der Schmelz-reduktionsprozesse bei Dauerbetrieb moglich gewesen ist, und bei der anderen hat es sich urn einen 3 t LD-Konverter gehandelt. Die Arbeitsweise beider Anlagen hat sich hinsichtlich der Gas-extraktion und der Geometrie der Behalter unterschieden, und wegen dieser Unterschiede ist man in der Lage gewesen, sich gegenseitig erganzende Untersuchungen in bezug auf die Gaser-zeugung und -zusammensetzung in der einen Anlage und die physikalischen Aspekte des Schlackenverhaltens wie z.B. Schaumen in der anderen durchzufiihren.

Wahrend des experimentellen Programms hat man Schlackenschaumen als ein potentielles ProzeBproblem identifiziert, das man mit einer Kombination der Verfahren gelost hat, die zur Modifizierung

15

der Faktoren konzipiert wurden, die die Erzeugung und Stabilisie-rung des Schlackenschaums steuern. Kleine Mengen oberf"lachen-aktiver Elemente, merklich Schwefel, sind zur Modifizierung der Oberflächenspannungseffekte benutzt worden. Man hat den Schlackenkieselerdegehalt wegen Herabsetzung des Schlacken-volumens minimiert und Basizitaten zwischen 2,0 und 2,5 wegen Reduzierung der Schlackenviskositat benutzt. Die Erzteilchen sind mit hoher Geschwindigkeit in den durch den Sauerstoffstrahl verursachten Hohlraum eingespritzt worden, urn das Vorliegen fester Chromeisenteilchen in der Schlackenphase zu minimieren. Diese MaSnahmen sind bei der Steuerung des Schlackenschaumens erfolgreich gewesen, aber es war nicht moglich, die Auswirkungen jeder MaBnahme in den Ergebnissen dieses experimentellen Pro-gramms zu isolieren. Man hat Schlackenschaumen als die Grenze fur Produktivitat eines konverterformigen Behalters identifiziert, und die in dieser Arbeit eingesetzten spezifischen Durchsatz-mengen haben dicht an dieser Grenze gelegen. Man hat gefunden, daB hohe Kohlenstoffkonzentrationen von 5% und mehr im Bad und Prozeßtemperaturen von mindestens 1600°C notig sind, urn eine akzeptierbare Chromausbeute von 90% und daruber zu gewinnen. Hohe Ausbeuten sind uber dem ganzen Bereich der Chromkonzentrationen erzielt worden, die fur Produktion eines rostfreien Vorproduktes zur Nachbehandlung erforderlich sind. Man hat eine eindeutige Korrelation zwischen den Kohlenstoffkonzen­trationen im Bad und der Chromausbeute gefunden und auch eine weitere Korrelation zwischen der ProzeBtemperatur und der Ausbeute, aber ein eindeutiger Effekt der Schlackenzusammen-setzung auf den Wirkungsgrad der Reduktion konnte nicht nachge-wiesen werden. Diese Beobachtungen zusammen mit der mikro-skopischen Untersuchung der aus den Experimenten entnommenen Schlackenproben sind zur Herleitung der wahrscheinlichen Reaktionsmechanismen benutzt worden. Man setzt voraus, daB die Reduktion der Chromeisenteilchen wegen eines diffusionsgesteuer-ten "schrumpfenden Kernmechanismus" stattgefunden hat, in dem Diffusion der Reduziermittel in die Chromeisenteilchen hinein und Diffusion der verschiedenen ionischen Spezies aus den Teilchen heraus zustande kommt. Man hat den im Metall gelosten Kohlenstoff als das wichtigste Reduziermittel identifiziert, und die bedeutendste Reaktion erfolgt an der Metal lteilchengrenzfläche. Dieser Mechanismus ist von dem oben erwahnten Einspritzen mit hoher Geschwindigkeit gefordert worden. In dem experimentellen Programm hat man auch die Erzeugung und Zusammensetzung der Prozeßgase untersucht, und man konnte zeigen, daB ein Gas mit gleichmaßiger Gute und Durchflußgeschwindigkeit erzeugt wird, das potentiellen Einsatz als ein Treibgas Oder zur Erzeugung von Elektrizitat finden konnte. Die Gaszusammensetzung ist von der in den Experimenten benutzten Kohlensorte abhangig gewesen, und es hat einen typischen Heizwert von 9,2 MJ/Nm3 ge-habt. Betrieb des Schmelzreduktionprozesses mit geschlossenen Abzugshauben wurde den Auffang dieses Gases und die Ruckgewinnung seines Heizwertes ermoglichen. Man hat die experimentellen Ergebnisse mit den Vorhersagen anhand eines mathematischen Warme- und MassenbilanzmodelIs des Prozesses

16

verglichen, urn die theoretische Verarbeitungszeit und den Roh-stoffverbrauch fur Produktion des Chromeisenproduktes im groStechischen MaSstab festzulegen. Die fur Umwandlung eines Chromeisenprodukts in einen rostfreiem Stahl benotigten Entkohlungs- und Verfeinerungsverfahren sind auch untersucht worden. Der hohe Kohlenstoffgehalt (5* Oder mehr) des Chromeisens wurde die Einblasperioden in konventioneller AOD-Verfeinerung auf ein unakzeptierbares Niveau ausdehnen, und er würde auch die ungekuhlte Abzugshaube einer typischen AOD-Anlage mit übermäßiger Warme belasten. Diese Probleme konnten gelost werden, wenn man eine Aufblasvariante des AOD-Verfahrens bis zum Ende des ersten Stadiums der Entkohlung adoptiert und zwar typisch 0,4% C. Ein mit einer LD-formigen Abzugshaube ausgeruste-ter Behalter ware notig, und wenn er dann im geschlossenen Modus arbeitet, waren weitere Energieeinsparungen durch Auffang der Prozeßgase im ersten Stadium der Verfeinerung moglich.

Steuerung des Phosphors ist bei der Herstellung rostfreier Stahle unbedingt erforderlich, aber zur Zeit sind keine Verfahren im groStechnischen MaSstab fur Entphosphorung des rostfreien Stahls vorhanden. Aus dem Grunde muB die Steuerung in der sorgfältigen Auswahl der Rohstoffeingaben liegen. Mit den Ergebnissen des experimentellen Programms hat man die erste flussige Roheisen-beschickung als die Quelle von uber 90% des zugeführten Phosphors identifiziert. Das deutet darauf hin, daB die bekannten Ent-phosphorungsverfahren fur flussiges Roheisen effektiv bei der Steuerung des Phosphorgehaltes im endgültigen rostenfreien Stahl-produkt sein konnten. In dieser Hinsicht ware die sorgfaltige Wahl der benutzten Kohlensorte auch vorteilhaft. Steuerung des Schwefels ist immer noch eine technische Unsicherheit in diesem Prozeß. Man hat diese Ergebnisse und Argumente zur Identifizierung der wichtigsten ProzeSparameter benutzt, die zur Produktion eines rostfreien Stahls mit dieser vorgeschlagenen Route erforderlich sind, und zwei Prozeßalternativen sind identifiziert worden. Im Falle der ersten wird ein Lichtbogenofen fur Schmelzen einer Kombination des einfachen und rostfreien Stahlschrotts gefolgt von Schmelzreduktion und Entkohlung ausgenutzt. Diese Alternative wurde maximale Verwertung des Chrom- und Nickelgehaltes im rost­freien Stahlschrott moglich machen. Die zweite Alternative betrifft Einsatz des Prozesses in einem integrierten Eisenhuttenwerk, wo das flussige Roheisen aus dem Hochofen vor Schmelzreduktion entsiliziert und entphosphert wird. Dies konnte entweder in einem zweckgebauten Behalter Oder in einem entsprechend modifizierten LD-Konverter durchgefuhrt werden. Am Konverter muBten Anlagen fur Kohle- und Erzaufbe-reitung, pneumatisches Einspritzen durch eine obere Lanze, Basalruhren mit tragem Gas und fur ProzeBgasauffang vorhanden sein. Verfeinerung des Chromeisens konnte entweder in einem danebenstehenden Konverter stattfinden oder möglicherweise in dem gleichen Behalter nach dem Entschlacken. Der Kapitalaufwand fur die eine oder andere Alternative hangt stark von dem bestimmten

17

Stahlwerk ab und auch von den dort bereits vorhandenen Einrich-tungen. Aus dem Grunde hat eine Einschatzung dieses Kapitalaufwandes außerhalb des Umfangs dieser Untersuchung gelegen. Im Vergleich mit dem ublichen UP-Lichtbogenschweißen im AOD-Verfahren deutet eine erste Bewertung der Rohstoffunkosten des vorgeschlagenen Prozesses auf potentiell signifikante Erspar-nisse hin, und deshalb empfiehlt man, daS eine eingehende wirtschaftliche Bewertung wegen Bestatigung dieser angedeuteten Ersparnisse unternommen wird.

18

Inhaltsverzeichnis Seite • , • 2 1

1. Einleitung 2. Theoretische Erwagungen 22

2.1 Reaktionmechanismen 22

2.2 Thermodynamische Erwagungen 23 3. Experimentelles Programm 24 4. Experimentelle Ergebnisse 25 4.1 Verhalten des Schlackenschaumens 26 4.2 Effekt des Metal 1 kohlenstoffgehaltes auf die 27

Reduktion von Chromeisen 4.3 Effekt der Prozeßtemperatur auf die

Reduktion von Chromeisen 4.4 Effekt der Schlackenbasizität auf die

Reduktion von Chromeisen 4.5 Mikroskopische Untersuchung der Schlackenproben

27

27 28

4 . 6 Gaserzeugung 28

5. Prozeßbeschickungsgleichgewicht 29 6. Produktion des rostfreien Stahls 30 6.1 Entkohlung 30

6.2 Steuerung des Phosphors und Schwefels 31 7. Erste wirtschaftliche Bewertung 31 8. Schlußfolgerungen 32

9. Literaturverzeichnis 34

Tabellen 35 Abbildungen 44

Anhang 64

19

Aufstellung der Tabellen 1. Experimentelle Parameter 2. Zusammensetzung der Rohstoffe 3. Analyse der TeiIchengroBe in den Rohstoffen 4. Zusammenfassung der Beschickung und des Ausstosses der Roh­

stoffe 5. Chrommassenbilanz 6. Spezifischer Rohstoffverbrauch 7. Rohstoffbilanz zur Produktion von 100 t Chromeisen 8. Rohstoffverbrauch im ersten Stadium der Entkohlung 9. Rohstoffunkosten der Schmelzreduktion

Aufstellung der Abbildungen 1. Liquidusoberfläche des CaO-MgO-Si02-Al2 03 Systems an der

20% AI2O3 Ebene zeigt die angestrebte Schlackenzusammen-setzung

2. Liquidusoberfläche des Eisen-Chrom-Kohlenstoffsystems 3. Hauptmerkmale der Versuchsanlage 4(a) C.S.M. (kontinuierliche Stahlherstellung) - Wärme 103 -

ProzeBzusammenfassung 4(b) C.S.M.-Warme 103 - ProzeBzusammenfassung 5(a) C.S.M.-Warme 104 - ProzeBzusammenfassung 5(b) C.S.M.-Warme 104 - ProzeBzusammenfassung 6(a) C.S.M.-Warme 105 - ProzeBzusammenfassung 6(b) C.S.M.-Warme 105 - ProzeBzusammenfassung 7. Mittlere Kohlenstoff- im Bad gegen die Chromruckgewinnung -

C.S.M.-Massenbilanz 8(a) C.S.M.-Warme 102 - ProzeBzusammenfassung 8(b) C.S.M.-Warme 102 - ProzeBzusammenfassung 9 Mittlere Badtemperatur gegen die Chromruckgewinnung

C.S.M.-Massenbilanz 10. Schlackenbasizitat gegen die Chromrückgewinnung 11. Mikrograph eines reagierenden Korns aus Experiment 105 12. Elementverteilung eines reagierenden Korns aus Experiment

105 13. Rohrgasanalyse von C.S.M.-Warme 81 14. C.S.M.-Warme 81, DurchfluBgeschwindigkeit des Abgasvolumens 15. Schmelzreduktion der Chromerze - Vergleich zwischen den

experimentellen Ergebnissen und dem berechneten Chromgehalt 16. Potentielle Verfahrenswege

20

THE PRODUCTION OF STAINLESS STEEL THROUGH SMELTING REDUCTION OF CHROME ORES USING COAL AND OXYGEN

British Steel Technical

ECSC Agreement No. 7210.AA/811

Final Technical Report

1. INTRODUCTION

The conventional route for the manufacture of stainless steel, in the European Community, is to melt charge chrome (FeCr), stainless and plain scrap, and for austenitic grades, nickel alloys, in an electric arc furnace, followed by decarburisation and refining in an oxygen blown process such as AOD or VOD. Charge chrome, which is the principal source of chrome units in the process, is an alloy of chrome, iron, carbon and silicon typically produced in a submerged arc electric smelting process, outside the Community and imported. The largest source of charge chrome is South Africa where the material is made from the large deposits of chromite ores in the Transvaal. Alternative, and potentially cheaper sources of chrome units are available, both within and outside the Community, for example in Greece and Turkey. The use of these materials may enhance the economics of production of stainless and alloy steels particularly if an alternative production route, avoiding the costly electric smelting and melting operations could be employed.

One such potential process had been developed by British Steel in its Teesside Laboratories for the reduction of iron ores, utilising smelting reduction technology*1). In that process co-products of liquid iron and a high calorific value gas were produced by the simultaneous injection of iron ore fines, granular coal, oxygen and fluxes into a liquid iron bath. Thermodynamic studies had shown the potential to reduce chromite ores in such a reaction scheme*2) and if this could be achieved in an industrial context then a possible coal-based route to stainless steel could be realised. The elements of such a process would be radically different from those of the conventional route and would comprise a smelting reduction step in which a liquid metal heel, for example blast furnace hot metal, would be enriched in chrome content up to the level appropriate for a stainless steel refining stock (typically 20% Cr), followed by a decarburisation and refining step in which additional coolants such as nickel and stainless steel scrap would be required to control temperature and composition.

The aim of this project was, therefore, to develop that process concept, at the pilot plant scale in order to establish the technical feasibility of the route. In addition the results of the pilot plant investigation would be related to the commercial scale of operation so that comparison of the energy requirements and economics could be made between the coal-based smelting reduction route and the conventional process*3'.

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2. THEORETICAL CONSIDERATIONS

The naturally occurring ores of chrome are spinel solid solutions with a range of mineral compositions, principally Fe304, FeCr204, MgCr204 and MgAl204. A study of the thermodynamics and kinetics of chromite ore reduction with carbon showed that the simplified reaction

Cr 2 0 3 + 3C -* 2[Cr] + 3CO (Reaction 1)

does not adequately represent the reduction of chromite ores and the following reaction schemes have been proposed*2*

Fe 3 0 4 + 4C -* 3[Fe] + 4CO (Reaction 2) (AG° = 663960 - 658.85T J/mol)

FeCr204 + 4C -+ [Fe] + 2[Cr] + 4CO (Reaction 3) (AG° = 1027260-688.52T J/mol)

MgCr204 + 3C -»(MgO) -I- 2[Cr] + 3CO (Reaction 4) (AG° = 863620 - 531.16T J/mol)

assuming in all cases that carbon is in excess.

Examination of the thermodynamic and kinetic aspects of these reactions, with particular emphasis on the effects of metal and slag composition and process temperature, was carried out in order to identify the most appropriate range of conditions for study in the pilot plant trials.

2.1 Reaction Mechanisms

Two principal, alternative reaction mechanisms have been proposed. In the first of these, derived from work on smelting reduction processes, grains of chromite must first be dissolved into the matrix of the slag where reaction between the chrome-containing components and solid carbon takes place*4.5). In this reaction scheme the dissolution step is quoted as rate limiting and dependent on solubility limits. Slag composition and fluidity are critical in controlling the solubility of chrome-containing species and the subsequent diffusion of the reactants and products.

The second mechanism, proposed by workers in the field of submerged arc charge chrome manufacture, is an ionic diffusion model(6>7\ in which the chromite grains reduce via a "shrinking core" mechanism. The postulate from these references is that reduction of chromite grains by carbon proceeds via the intermediate of CO diffusion into, and diffusion of a variety of ions out from, the grain. The iron-containing minerals are reduced first, according to this mechanism. Once this step is complete, reduction of the chromium ions takes place, leaving an MgO-rich shell which ultimately will dissolve into the matrix of the slag, again subject to solubility limits.

The work reported in the smelting reduction field(4>5) was considered to be closely analogous to this proposed process route and so slag composition was identified as a key parameter for the pilot plant investigation. Slag fluidity would be essential to promote dissolution into, and diffusion of the chrome-containing species through the bulk slag. In addition it had been reported that the kinetics of reduction of chromite by this proposed reaction mechanism would be severely inhibited by the formation of a solid layer of MgAl204 on the surface of chromite particles at AI2O4 levels above 21%(4>. Further constraints on the acceptable range of slag compositions were posed by the basic refractory lining of the pilot plant vessels to be used for this work.

22

These considerations were used to select the aim slag composition range for the experimental programme, which was a compromise between the achievement of good fluidity at a low liquidus temperature, and compatibility with the refractory lining. Fig. 1 shows part of the liquidus surface in the CaO-MgO-Si02-Al203 system, at the 20% AI2O3 plane, and indicates a composition to achieve a liquidus temperature of approximately 1400°C at 35% CaO, 35% SiC>2,10% MgO and 20% AI2O3, with a complex basicity of 0.82. This level of basicity is lower than that normally used with the magnesia refractory linings of the pilot plant vessels and was selected as the lower limit for the experimental programme, with the upper limit chosen as the more normal 2 to 3 for this type of lining. The high AI2O3 levels inherent in chromite ores, typically up to 16% in Transvaal material, meant that achievement of the required slag AI2O3 levels below 21% must be by dilution with mixed fluxes of lime and silica, in proportions chosen to achieve the desired slag basicity. This practice would lead to high slag bulks in the process.

2.2 Thermodynamic Considerations

Thermodynamic studies indicate that the product of the reduction reactions will not be elemental chrome, but a series of stable chrome-carbides where composition is temperature dependent. At about 1150°C Cr3C2 forms, with a carbon content of 13.3%, between 1250 and 1600°C O7C3 would be formed and between 1600 and 1800°C C ^ C g would form (with carbon contents of 9.0% and 5.7% respectively). Elemental chrome would only be formed at temperatures above 1800°C. The result of the formation of these carbides is that at a typical smelting temperature of 1600°C, the activity of carbon in the Fe-Cr-C system is depressed, and is reported between 0.1 and 0.3 in the region of chrome concentrations of 0% to 20% and carbon between 2% and 4%. Increasing the carbon concentration to between 5% and 6% would increase its activity to between 0.6 and 1.0 (at the carbon saturation level)(2>. In addition, carbon content of the metal phase has a significant effect on its liquidus temperature, as shown in Fig. 2, which indicates a steep rise in the liquidus as carbon levels rise above 4%. Carbon concentration in the metal was, therefore, identified as a second key parameter for the experimental programme, and two distinct ranges were studied. The first range was between 2% and 4% C which would give a low liquidus temperature, but also a low thermodynamic activity. Under these conditions carbon dissolved in the metal would not be effective as the reducing agent, leaving free carbon in the slag phase to act in this capacity, as reported by other workers*4). The second range to be studied would be close to saturation levels, from 4% to 6% or higher, which would increase the reducing potential of the system. However, in the context of the overall process aims of the production of stainless steel, the higher carbon levels in the metal of between 5% and 6% would be undesirable because of the greater load imposed on the subsequent decarburisation and refining step.

In addition to slag composition and bath carbon content, process temperature was identified as a key parameter. Thermodynamic studies show that in addition to its affect on the carbon content of the products of reduction, process temperature has a marked effect on the reduction reactions. Between 1200 and 1400°C the Fe3C<4 and FeCr204 will reduce, but temperatures above 1500°C are necessary to reduce the MgC^CU components*8) and thus achieve the necessary high chromium yields. In addition higher temperatures will enhance the overall kinetics of the reactions and so the experimental programme was designed to investigate a range of process temperatures between 1500 and 1650°C.

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3. EXPERIMENTAL PROGRAMME

In addition to the three key parameters of slag composition, bath carbon content and process temperature identified from theoretical arguments, other factors were considered to be critical to the success of the process. These factors were the composition of the ore and coal, the method of feeding of the ore and coal, the physical properties and behaviour of the slag and the degree of mixing both in the slag phase and between the metal and slag.

The experimental programme was designed to study the effects of these factors, within the ranges of the key parameters already identified. A total of thirty experiments was performed, the experimental conditions of which are summarised in Table 1, sorted according to the ranges of the key parameters. Table 1 contains the Experiment and Heat (vessel) numbers to allow reference to the greater detail given in the individual semestrial reports.

The experiments were carried out using two pilot plants at British Steel's Teesside Laboratories, each with a nominal 3 tonnes capacity, which share common control and instrumentation systems, enabling the same process measurements to be taken in experiments on either plant. One was a purpose-built pilot plant designed to enable studies of continuously operating smelting reduction processes to be carried out, and the other was a 3 tonne scale BOS-type converter. They differed in their modes of operation with respect to gas extraction, and in the geometry of the vessels. These differences enabled complementary studies to be carried out of gas generation and composition on one plant, and of physical aspects of slag behaviour such as foaming, on the other. The detailed descriptions of the plants and the differences between them are contained within the semestrial reports*3). Fig. 3 shows a schematic representation of the smelting reduction pilot plant and illustrates the significant features of the chrome smelting step in this proposed process route to stainless steel. Oxygen was blown onto a liquid iron bath, in a manner similar to BOS steelmaking practice, so that reaction between oxygen and carbon took place to generate heat. Coal was also blown, from above, to replenish the carbon in the metal bath. Chrome ore and additional coal, as the reductant, were added from above with the thermal requirement for the reduction reactions being supplied by the oxygen-carbon reactions. Careful manipulation of the oxygen, coal and ore addition rates enabled control of the metal carbon concentration and temperature, whilst the chromium content rose, with time, as the result of the reduction reactions. Control of slag composition was by addition of fluxes, in lump form, from above by gravity. Mixing of the vessel contents was achieved by the injection of inert gases through the vessel base, together with the mechanical energy supplied by the high velocity oxygen, coal and ore jets.

Both vessels were equipped with facilities to take metal and slag samples and bath temperature measurements, so that the progress of the reactions could be monitored throughout each experiment. The vessel illustrated in Fig. 3, was operated in a suppressed combustion mode, in which the vessel and its hood were mated together and sealed against air ingress, enabling sampling of the gas generated in the process to be made at a convenient point in the gas off-take ductwork, close to the vessel. The gas sample was passed to a mass spectrometer for continuous analysis of its composition. The alternative plant, the 3 tonne scale BOS converter, was operated in the open hood mode, in which the gas was fully combusted at the vessel mouth prior to extraction in the gas ductwork. In both cases the gas flow in the ductwork was measured by a venturi section.

Each plant was equipped with pneumatic powder feeding facilities to enable injection of mixtures of coal and ore powders, at individually controlled rates. This mixture could be injected through a central port in the oxygen lance on each plant, but in addition, the converter pilot plant had a second, auxiliary lance through which the powder feeds could be injected enabling an investigation of the process effects of raw material placement to be carried out. Each plant also had facilities to

24

add lump material such as fluxes or briquetted ores, at controlled rates by gravity. The use of these alternative feeding techniques is shown in Table 1.

The chemical compositions of the raw materials used in the experimental trials are shown in Table 2, and the particle size data given in Table 3.

The experimental technique for either vessel was the same. The vessel was preheated prior to introduction of the initial charge of hot metal, which was supplied from an electric arc furnace and either tapped directly into the vessel, or transferred via an intermediate ladle. The composition of the initial metal charge was closely controlled according to the aims of the experiment, with the critical components being its carbon, chrome and silicon contents, together with its temperature. In some cases small quantities of arc furnace slag were also transferred to the vessel with the initial charge, although this was avoided where possible. The quantities of this slag are shown in Table 4. Weights and samples of metal and slag were taken prior to the start of oxygen blowing. Each experiment began with the oxygen flow, and once reaction had been ignited (within the first 15 seconds), coal additions commenced and a batch of lime added to form the initial slag with the silica generated by oxidation of the silicon content of the hot metal charge. Coal and oxygen blowing would continue until the desired conditions of process temperature and bath carbon content had been attained, detected by the metal samples taken at regular intervals. Once these conditions had been reached, the chrome ore addition would be started and maintained at rates to keep the process temperature within specification. Raw materials inputs and exhaust gas composition and flow were monitored continuously, and regular samples of metal and slag were taken throughout each experiment. Each experiment would continue until either the raw materials were exhausted, control over process conditions was lost, or some engineering feature of the plant caused a shutdown.

Engineering constraints on the plant did limit the total duration of an experiment to approximately 3 hours, which was reached on several occasions. This meant that no experiment would be expected to take the chromium content of the metal from zero to the 20% level required for stainless steelmaking, and so the programme was designed to study process operation in bands of chromium content, from 0 to 10%, and 15% to 20% as shown in Table 1.

At the end of each experiment further samples of metal, slag and fume were taken, together with bath temperature measurements. All metal, slag and fume generated were collected and weighed, although in some cases it was not possible to remove all materials adhering to the vessel walls, which remained in place at the start of the next experiment and would cause a discrepancy in the mass balances for each experiment.

4. EXPERIMENTAL RESULTS

The detailed results of all experiments performed in this study are reported, in both tabular and graphical form, and in chronological order, in the semestrial reports*3*. The first three experiments performed were designed to commission novel aspects of pilot plant operation necessary for this work. Those commissioning experiments used ferrous feedstocks and their results are excluded from those shown in this report, they were however fully reported in the first semestrial report*3). For the remaining chrome ore smelting experiments summaries of the raw materials consumptions and the chromium mass balances are given in Tables 4 and 5, in each case sorted according to the values of the key parameters as shown in Table 1. The Heat Numbers are given in these Tables to allow reference back to the detailed results contained in the semestrial reports. Table 5 shows that for many of the experiments there was a loss of chrome units in the mass balances, although in some cases there was a gain, which was attributed to the material

25

adhering to the vessel walls, as previously discussed. These discrepancies in the mass balances caused uncertainties in the calculation of chromium yields, as shown in Table 5. Two values of yield are quoted, one is calculated by dividing the chromium reporting to the metal by the total chromium input, the other by dividing by the total chromium output. The difference between the two values is a measure of the accuracy of the mass balance. No mass balance was calculated for heat number 77 because of its short duration and the low quantity of chrome ore added.

Examples of the experimental results are given in graphical form for three experiments in Figs. 4-6 (a and b). These figures show the changes in bath carbon and chrome content, and temperature together with raw material feed rates and slag compositions. The experiments selected had high yields of chromium in the metal over the full range of bath chromium contents (Figs. 4 and 5), and an example of the reduction of both South African and Greek chromites is shown (Figs. 4 and 6).

4.1 Slag Foaming Behaviour

The first set of experiments, numbers 76 to 91, in this work were carried out using the purpose-built Continuous Melter pilot plant. Several of these had to be terminated following sudden, uncontrolled evolutions of gas. In particular experiment 77 had to be shut down after only 6.3 minutes of chromite ore feeding, which was insufficient time to allow useful data on the reduction of the ore to be obtained. These events appeared to be associated with the generation of highly viscous slags which may have impaired the mixing between oxygen and carbon. Lowering of the slag basicity and increasing the stirring gas flow improved the slag fluidity and mixing, but generated slag foam which filled the limited freeboard volume available on that plant. The subsequent experiments, numbers 92 to 107, were carried out on the 3 tonne scale BOS converter pilot plant, whose greater freeboard and open hood operation enabled better observation and containment of any slag foams generated. Those experiments clearly demonstrated the severity of the slag foaming problem, with large volumes of stable foams being generated within a short time of the start of chrome ore feeding. Early studies of slag foaming behaviour had identified the importance of surface tension, viscosity and the presence of solid particles on the formation and stabilisation of slag foamsW. Actions wee taken to modify each of these factors in order to control the slag foaming problem.

A small increase in the sulphur content of the liquid metal charge, from a range of 0.026% to 0.058% in the earlier experiments, to between 0.053% and 0.17% in the later experiments, was used to modify the surface tension effects that govern the formation of slag foams. It is known, from BOS steelmaking practice, that sulphur is a surfactant and that it is effective in very small quantities in the destruction of slag foams generated in that process.

Stabilisation of foams is governed by viscosity effects which control the drainage of the liquid component of the foam into the bulk slag. The true viscosity of slags in the CaO-Si02-Al203-MgO system tends to rise with increasing silica and alumina contents. The alumina content of the slags in this process are controlled mainly by the mineral content of the ore, but the silica content of the slags in the early part of these experiments was controlled by the silicon content of the initial hot metal charge. This was reduced to between 0.075% and 0.35%, compared with a typical value of 0.7% in the early experiments. This action would not only have reduced the viscosity of the early slag, but would also have reduced its total quantity by requiring less flux additions to make the specified basicity.

The presence of solid particles tends to stabilise slag foams by modifying the effective viscosity of the liquid, by residing in the interfacial film between the gas bubbles, thus blocking the drainage paths. A variety of ore feeding techniques were used in these experiments, as shown in Table 1.

26

Injection of ore powder via the auxiliary lance introduced the particles directly into the slag phase, and those experiments all generated large quantities of stable foams.

The small increase in sulphur, the increased basicity and reduced volume of the slags, together with the use of briquetted ores or the high velocity injection of ore powders through the central port of the oxygen lance were effective in controlling the tendency of the slag to foam, but it was not possible to identify the individual effects of these actions from the results of these experiments.

4.2 The Effect Of Metal Carbon Content On The Reduction Of Chromite

Fig. 7 shows the calculated chromium yields to the metal phase as a function of the typical bath content of each experiment. As previously explained, the discrepancies in the mass balances, shown in Table 5, led to the calculation of an upper and lower value of the yield for each experiment, and this is represented by the vertical bars in the figure, which also contains a line, calculated by linear regression, which shows a clear rising trend of yield with carbon content. This is also shown in Tables 1 and 5, where those experiments with yields greater than 80% were all those carried out with high bath carbon contents. Table 1 also shows that those experiments were also carried out with high slag basicity, leaving the possibility that the high yields were the result of that key parameter, or a combination of both. Table 1 shows that experiment number 96 was carried out at high basicity, stirring rate and temperature, but at low bath carbon content and the resulting yield was only 54.9%, taking the average between the minimum and maximum calculated yields. In addition, the results of experiment number 102, shown in Fig. 8 (a and b), demonstrate that reducing conditions were achieved with rising carbon contents, but not once the metal carbon concentration fell. These results confirm the influence of metal carbon content on the yield of the reduction reactions, and show that acceptable yields of 90% or better were only achieved at carbon levels greater than 5%.

4.3 The Effect Of Process Temperature On The Reduction Of Chromite

No clear correlation between process temperature and yield could be found by studying the results of all of these experiments in a single group, which is due to the masking effect of bath carbon content. Table 1 shows that the experiments with high temperatures also had low, or medium carbon concentrations, and this is reflected in their low yields. However, Fig. 9 which shows the results only of those experiments performed at high bath carbon content, does show a rising trend of yield with process temperature, as expected from theoretical considerations. The figure demonstrates that for acceptable yields a minimum process temperature of 1600°C must be maintained.

4.4 The Effect Of Slag Basicity On The Reduction Of Chromite

The theoretical considerations discussed earlier in this report indicate that slag fluidity would be a key parameter in the experimental work. In particular, if the reaction mechanism involved the dissolution of chromite particles prior to reaction, then slag fluidity would be crucial to the efficiency of the reduction reactions. Fig. 10 shows the yields of the experiments plotted as a function of slag basicity, and this appears to show a positive correlation. However, as previously discussed, this result is distorted by the effects of high bath carbon content, which was the dominant factor in these experiments. No clear effect of slag composition on reduction efficiency could be found from these experimental results. However, high slag basicity was one of the factors used to control slag foaming, and it is concluded that this is the true significance of slag basicity for this process. It is recommended that a complex basicity of between 2.0 and 2.5 be used for this purpose.

27

4.5 Microscopic Examination Of Slag Samples

Samples of slags taken from experiments carried out in the third and fourth semesters were examined using both optical and electron microscopy techniques, with the objective of providing information on the mechanisms of the reduction reactions. The results of that investigation are reported in detail in the fourth semestrial report and are illustrated by Figs. 11 and 12, which show a reacting grain in a sample of slag taken from experiment 105. This sample, which was typical of all the samples examined, shows that the slag contained solid chromite grains in various stages of reduction and small droplets of reduced metal. The grains were being reduced by a "shrinking core" mechanism, as indicated by the element distribution map of Fig. 12. This shows the boundary of a grain at a late stage in its reduction with an MgO-rich residual shell. The bulk slag phase is principally a CaO-SiC>2 composition, with AI2O3 from the grains dispersed through it. The particle is surrounded by small droplets containing chromium and iron. This observation is typical of a diffusion controlled reaction, with both diffusion of the reductant into and diffusion of the various ions out from the core of the particle taking place, and is similar to the mechanism reported by the workers investigating the submerged arc furnace process^. None of the samples examined showed any evidence of the dissolution of chromium-containing species into the bulk slag prior to reduction, ruling this out as a possible reaction mechanism.

Any of the possible reactions to produce iron and chromium by reduction with carbon, carbon monoxide or hydrogen, would produce a gaseous product, either CO, CO2 or H2O, but none of the samples examined showed any clear evidence of gas bubbles in the vicinity of the chromite grains. There are three possible explanations for this, either the bubbles were too small to be seen at the magnifications used, or they escaped whilst the sample was cooling, or the particles observed were no longer at the reaction site and the reaction had been suspended.

As with gas bubbles, the samples examined showed little evidence of graphite grains in the slag. If the reaction sites were in the bulk slag, then the most likely reductant would be CO and H2 from the volatile component of the coal. In that case the activity of the reductant would be related to the gas composition and production rate, which would be functions of the coal and oxygen input rates, and not of the bath carbon content as observed.

It is deduced from these observations that the reaction site was not in the bulk slag, as originally believed, but at the particle-metal interface. At this site the availability of reductant, in the form of carbon dissolved in the metal phase would be high, particularly in those experiments carried out at close to saturation levels. Reaction would still proceed by the diffusion mechanism reported above, but some particles would be swept away from the reaction site by turbulence and bulk movement in the slag phase, and would remain frozen in their current state of reduction until they were once more transported to the reaction site. It is postulated that these were the particles observed in the slag samples. The mechanism would be aided by the high velocity injection of the ore particles into the jet cavity. With this mechanism the basicity of the slag is of secondary importance, as its role would be to absorb the MgO and AI2O3 residues of reduction, rather than to dissolve the reacting minerals or to provide an effective mass transfer medium for the reacting species. This is borne out by the observed lack of influence of slag basicity on the reduction of the chromite ores.

4.6 Gas Generation

Previous work on iron-bath smelting reduction had shown the value of the gas stream in the overall process economics* *>, with a clear requirement to achieve a gas of consistent quality for subsequent sale or use. Gas composition and flow rate were monitored throughout these experiments, and sample results are shown in Figs. 13 and 14. These show that a high calorific

28

value gas was generated, with a typical composition of 56% CO, 17% H2,7% CO2,7% N2, and the balance to 100% being argon which was used in these experiments as the pneumatic transport gas for the coal and ore, so that any air ingress into the vessel could be quantified by nitrogen balance. Any commercial process would use air as the carrier gas, and the gas composition must be recalculated to account for this. Two alternative assumptions can be made about the behaviour of the oxygen component of the carrier air, it could either react with carbon in the same manner as the primary oxygen, or it could react with CO to form CO2. The results of these calculations show that in the first case, the gas composition would be 57.6% CO, 17.5% H2,7.2% C0 2 and 17.8% N2, with a net calorific value of 9.2 MJ/Nm3, and in the second case the composition would be 52.0% CO, 17.5% H2,12.8% C02 and 17.8% N2, with a NCV of 8.46 MJ/Nm3. The measured flow rate of gas shown in Fig. 14 had a typical value of 1825 Nm3/h, which would become 1775 Nm3/h as a result of the above calculations.

The gas contained fume which was collected at the bag filter unit in the gas treatment plant and weighed for each experiment, as shown in Table 4. The fume loading varied widely between experiments, which was, in part, due to problems in operation of the bag filter. These experiments show that the fume loading to be expected in the process gas would be in the range 80 to 100 g/Nm3, which is typical of other top blown smelting reduction and steelmaking processes*1*.

5. PROCESS CHARGE BALANCE

The results of this experimental programme have identified the ranges of the key process variables for efficient reduction of chromite ores. Those ranges are:

Bath Carbon 5% or greater Process Temperature 1600°C or greater Slag Basicity 2 to 2.5

These values were used in a predictive heat and mass balance mathematical model of the process to establish the theoretical processing time, raw materials consumptions and production rates to generate a metal suitable for further processing to stainless steel. These values were compared with the experimentally determined rates to achieve the specified conditions. The results of the calculations and the comparison with experimental data are shown in Table 6. The experimental data in the Table were taken from heat 105 and show the average consumptions over the entire heat which are affected by heat losses. These were higher at the pilot plant scale than would be expected for a production scale vessel. The heat losses in the early part of any experiment would be high because of the heating of the refractory from the preheat temperature of around 1100°C to the process temperature. Later in the experiment this effect would be less significant and additions rates were adjusted to reflect this and maintain steady temperatures. Those rates would be more typical of a production scale process, and are also quoted in Table 6.

The calculated process time shown in the Table is 360 minutes to produce a metal of 18.6% Cr. The highest chromium enrichment rate achieved in the experiments was 0.061% Cr/min at the input rates shown in Table 6. This rate would not be maintained throughout the duration of the enrichment process, even at constant input rates. This is because the metal produced by the reduction of the ore would be at a composition determined by its chrome-iron ratio, and from an initial starting point of zero chrome in the metal heel, the metal composition would approach that value in an asymptotic manner. This effect is shown in Fig. 15, which shows the calculated chromium concentration resulting from constant ore additions at similar rates to those employed in experiments 103 and 104. The results of those experiments are superimposed on the figure, with the data from experiment 104 displaced in time to fit the curve. The figure suggests a process

29

time of between 400 and 450 minutes to reach 18% Cr, but those two experiments were operated at slightly lower chromium input rates than that of experiment 105.

Processing time could be decreased by an increase in the ore additions rate, but this would require an increase in the oxygen and coal input rates and would generate increased quantities of gas, which would make the problem of foam generation worse. The experimental results showed that the limit to the productivity of the process would be caused by excessive foam generation, and that the specific input rates used were close to that limit for a typical converter shaped vessel.

The data of Table 6 were used to calculate the materials balance to produce 100 tonnes of chrome-iron, as shown in Table 7.

6. THE PRODUCTION OF STAINLESS STEEL

In order to produce stainless steel from the product of the smelting reduction step, the metal must be decarburised and refined. The major constituent to be removed would be carbon, as already identified, but the minor components of phosphorus and sulphur would also be of critical importance to the achievement of stainless steel product specifications.

6.1 Decarburisation

The conventional method for the decarburisation of the crude stainless steel tapped from the electric arc melting step in the current process route, is the AOD process. In essence that process is an oxygen refining system in which oxygen is introduced through side blowing tuyeres located near the vessel base. The AOD process proceeds in discrete stages with varying ratios of oxygen and inert gas through the tuyeres, whilst maintaining a constant overall flow of gases. Process conditions used during AOD decarburisation vary according to the grade of stainless steel produced and a typical first stage practice uses a ratio of oxygen to inert gas (usually nitrogen) of 5:1 and continues until the bath carbon content is nominally 0.4%. Subsequent stages use increasing proportions of inert gas (normally argon) until the specified carbon level is attained.

The carbon content of the liquid metal charge to the AOD depends upon the charge mix employed in the arc furnace and may be in the range 0.8% to 2.5% C. With typical stage one specific blowing rates of 0.8 Nm3 02/min/tonne, decarburisation times of 10 to 30 minutes are required to reach the 0.4% carbon level at the end of that stage. The metal generated in the smelting reduction step would contain a minimum of 5% carbon, which would extend the blowing time of the conventional AOD process to 65 minutes or more. Top blown variants of the AOD process have been developed^10) in which oxygen blowing rates similar to those employed in BOS steelmaking have been achieved in the initial part of the decarburisation whilst the carbon content is above 0.4%, this would reduce the stage one processing time to between 15 and 20 minutes. At carbon levels below 0.4% the process would revert to conventional AOD practice.

Heat is liberated in the decarburisation stage, and coolants must be employed to control the process temperature and composition. In AOD decarburisation coolants must not be added too early in the blow to avoid low temperatures which favour the oxidation of chrome. The conventional practice is to add coolants in lump or granular form from bunkerage above the vessel. The use of stainless steel scrap would entail interruption of the blow in order to charge from a pan.

In the decarburisation of the chrome-iron generated by this smelting reduction process nickel must be added to achieve the 8% level necessary for typical austenitic grades of stainless steel. This could either be in the form of granulated ferro-nickel alloys or nickel oxide. The choice would

30

depend upon price and availability. The high carbon content of the chrome-iron would require additional coolant in the form of stainless steel scrap.

Table 8 shows the calculated raw material consumptions for the decarburisation step. The Table also shows the quantity of gas that would be generated. Conventional AOD plants are operated in the open hood mode in which the CO gas is fully combusted at the vessel mouth and temperatures in the uncooled hood are controlled by utilising excess air in the combustion. Operation at the carbon levels expected in this process would impose an unacceptable heat load on the hood, and would also represent a significant loss of energy. In order to handle the quantity of gas generated it would be necessary to utilise a water-cooled hood, such as that used in BOS steelmaking. Operation in closed hood mode with a gas collection plant would enable recovery of the chemical energy in the gas generated during the first stage of decarburisation, where the metal carbon content is reduced from 5% to 0.4%. At carbon levels below this, when the gas must be diluted by increasing volumes of inert gases, gas collection would not be recommended.

6.2 Control Of Phosphorus And Sulphur

There is no technique currently available for the removal of phosphorus from stainless steel, and phosphorus specifications are becoming increasingly stringent. Current practice is to select raw materials with great care to avoid phosphorus problems, and coal is known to be a potential source of phosphorus. The results of the experimental work of this study showed that the phosphorus content of the tapped metal was lower than that of the initial hot metal heel, and mass balance calculations showed that the hot metal charge accounted for 93.3% of the phosphorus inputs. The phosphorus was distributed 67.1% to the final metal, 15.2% to the slag, 8.7% to the fume, with a loss of 8.9% of the input, possibly to the gas stream. It is clear from these results that control of phosphorus content in the final stainless steel must be by control of the phosphorus burden of the initial hot metal charge, which could be achieved by known hot metal dephosphorisation techniques prior to chromium enrichment. Selection of the coal to minimise phosphorus inputs would also be beneficial.

The sulphur content of the metal produced in these experiments was high, with typical end of blow levels between 0.035% and 0.1%. This was partly because of sulphur pick up from the coal, and partly because of the deliberate addition of sulphur to control slag foaming. The high slag volumes generated during the smelting reduction step would be beneficial in the control of sulphur, and further optimisation of the slag composition may be possible to improve its sulphur capacity.

7. PRELIMINARY ECONOMIC EVALUATION

The previous sections of this report have identified the raw material consumption and processing times required to produce a crude stainless steel composition utilising this smelting reduction technology. A full economic evaluation of the production route would require details of the capital cost requirements to implement the process, and these would be strongly site specific, and as such are beyond the scope of this report. The preliminary economic evaluation carried out for this study was, therefore, confined to an examination of the raw material costs for the production of crude stainless steel by this route, and some important factors, notably refractory costs, have been omitted. The cost data used are considered to be typical of a European steelworks, and are not specific to any one site. Where possible, price data have been taken from the 'Metal Bulletin' in October, 1990 and converted to pounds sterling at the current exchange rates.

Table 9 shows the raw material costs for the smelting reduction route up to the end of the first stage of decarburisation. This shows a calculated raw material cost of £472 per tonne of crude

31

stainless steel. Comparison with existing production routes for the manufacture of stainless steel would, like capital costs, be site specific and such data are not readily available, being of a confidential commercial nature. However, a simple comparison of the price differential between chrome units in the form of chrome-iron and as charge chrome shows a factor of approximately 2:1 in favour of the chrome-iron, which clearly demonstrates that this smelting reduction technology represents a significant opportunity to minimise costs.

There are process options which would allow advantage to be taken of this opportunity either at an electric arc furnace based plant, or at an integrated iron and steelworks, see Fig. 16. Each of these options would have different technical and economic characteristics, which must be studied in detail for each possible application of the process.

The electric arc furnace based route would use the EAF to melt a combination of plain and stainless steel scrap followed by the smelting reduction and refining stages as outlined in this report. The advantage of this route is that it would enable maximum use to be made of stainless steel scrap to recycle its chromium and nickel content.

The alternative mode of operation would be operable on an integrated iron and steelworks utilising modifications to an LD type of steel plant. It is envisaged that the smelting reduction step would take place in one vessel and refining in an adjacent one, although it may be possible to use the same vessel for both operations, with intermediate deslagging. It would be necessary to avoid the possibility of contamination of the other steel products with chromium and nickel, and this would be achieved if the stainless steel were made in a vessel near the end of the life of refractory lining.

8. CONCLUSIONS

1. The experimental programme carried out in this work has demonstrated the technical feasibility of the production of a chrome-iron by the smelting reduction of chrome ores with coal and oxygen. High metal carbon contents, 5% or greater, and high process temperatures, 1600°C or greater, were identified as the key process parameters required to achieve high yields of chromium over the full range of compositions required for stainless steelmaking.

2. Problems of slag foaming were encountered in the experimental work, and techniques to control this behaviour were developed. High basicities were used to reduce the viscosity of the slag. Reduction of the silicon content of the initial hot metal charge was beneficial in maintaining high basicities and minimising flux consumption and slag bulk. Containment of slag foams has been identified as the limit to productivity for a typical converter shaped vessel.

3. Examination of slag samples by optical and electron microscopy techniques has indicated that the reduction reactions take place via a "shrinking core" mechanism with diffusion of reductants into and ions out from the solid chromite grains, leaving an MgO-rich shell to be assimilated into the bulk slag phase. The most probable reaction site has been identified as the metal-chromite grain interface, with carbon dissolved in the metal phase as the major reductant.

32

4. High velocity injection of mixtures of coal and chromite particles into the oxygen jet cavities resulted in the highest yields obtained. This technique maximised the contact between ore particles and the bulk metal phase and is consistent with the reaction mechanism proposed above.

5. Gas of consistent quality and flow rate was produced in the experiments, with a composition dependent upon the coal used. A gas with a typical calorific value of 9.2 MJ/Nm3 would be generated during this process.

6. These results of this work have been used to identify the key process elements required to produce stainless steel by this proposed route. Two alternative process options have been identified. One would utilise the electric arc furnace to melt a combination of plain and stainless steel scrap, followed by the smelting reduction and decarburisation steps.

The second option would be to operate the process in an integrated iron and steelworks, in which the blast furnace hot metal would be desiliconised and dephosphorised prior to the smelting reduction step. This could be carried out either in a purpose-built vessel, or in a suitably modified BOS converter.

7. The experimental results have been related to the predictions of a heat and mass balance mathematical model of the process in order to generate a charge balance and process rate model. This showed that processing times of 360 minutes would be required to enrich an initial charge of blast furnace hot metal from zero chromium to levels required for subsequent refining to stainless steel. Proportionately lower processing times would result for the arc furnace route, depending upon the chromium content of the feed into the smelting process.

8. The high carbon content of the product metal, 5% or greater, would require excessive blowing times for refining by conventional AOD practice. The use of a top blown variant of the AOD process would be effective in reducing the refining time back to conventional levels.

9. Control of phosphorus is essential in stainless steelmaking, and the experimental results showed that the most effective method, in this process route, would be by control of the phosphorus content of the initial hot metal charge. This requirement, together with that of minimising the hot metal silicon content for slag control, indicates that hot metal desiliconisation and dephosphorisation may be essential steps in the route based on blast furnace iron. Control of sulphur in the smelting reduction and refining steps remains a technical uncertainty in this work.

10. Preliminary examination of the raw material costs for the process route indicate a significant potential saving compared with the conventional electric arc melting and AOD refining route. These results highlight the requirement for an in-depth analysis of the economics of this process route.

33

9. REFERENCES 1. RobsonAL

Co-Production of Liquid Iron and Synthesis Gas in Smelting Reduction Commission of the European Communities Report EUR 12061 EN

2. HealyGW The Thermodynamics of Chromite Ore Smelting Trans ISS, 1988,9,153-161

3. Treadgold C J The Production of Stainless Steel Through Smelting Reduction of Chrome Ores Using Coal and Oxygen Semestrial Reports 1 to 4, July 1988 -June 1990 ECSC Agreement No. 7210.AA/811

4. Katayama H G, Satoh M, Tokuda M Fundamental Study on Smelting Reduction of Chromite Ore Powder Tetsu-to-Hagane, 1988,74, (12), 2361-2363

5. Fujita M et al Smelting Reduction of Chrome Ore Pellet in Stirred Bath Tetsu-to-Hagane, 1988,74, (4), 680-687

6. Rankin J R The Composition and Structure of Chromite During Reduction with Carbon Arch Eisenhuttenwes, 1979,50, (9), September, 373-378

7. Perry K PD, Finn CWP, King R P An Ionic Diffusion Mechanism of Chromite Reduction Metallurgical Trans. B, 1988,19B, August, 677-684

8. HealyGW Carbon Reduction of Chromite in Bird River and Other Ores and Concentrates at 1200-1700°C Can. Met. Quart 1988,27, (4), 281-285

9. Kozakevitch P P Foams and Emulsions in Steelmaking Journal of Metals, July 1969,57-68

10. HeinkeRetal Developments in Refining Stainless Steel Melts by the Krupp Combined Blowing Process International Oxygen Steelmaking Conference, Austria 25-28 May 1987

34

TABLE 1

EXPERIMENTAL PARAMETERS

171

Experiment Number

1 2 3 27 26 28 30 10 29 25 9 13 22 23 24 7 4 12 14 21 6 8 5 19 11 15 16 20 17 18

Range Definition

Heat Number

73 74 75 104 103 105 107 87 106 102 82 90 99 100 101 80 76 89 91 98 79 81 77 96 88 92 93 97 94 95

HIGH MEDIUM LCW

Bath Carbon

%C < > <

HIGH HIGH HIGH HIGH HIGH HIGH

MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM LCW LCW LCW LOW LCW LCW LCW %C

4.0-6.5 2.0-4.0

<2.0

Bath Temperature

Deg C

MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM LOW HIGH HIGH HIGH

MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM MEDIUM LCW LCW LCW HIGH HIGH HIGH HIGH HIGH

MEDIUM MEDIUM Deg C

1600-1700 1550-1600 1500-1550

Slag Basicity

HIGH HIGH HIGH HIGH MEDIUM MEDIUM MEDIUM LCW LCW

MEDIUM MEDIUM MEDIUM LOW LOW LOW LCW LOW LOW LOW LOW HIGH LOW LOW LOW LOW

MEDIUM LOW

2.0-3.0 1.5-2.0 1.0-1.5

Bath Chrome Range %Cr

Stirring Rate

mMMT^QTfYJTM'i ui?RTe \*\Jl11UuulU

HIGH MEDIUM MEDIUM MEDIUM LOW

MEDIUM LOW LOW LOW LOW LCW LCW HIGH LOW LOW LOW LOW HIGH HIGH LOW LOW LOW LOW LOW LCW LOW LOW %Cr

10-20 5-10 <5

HIGH HIGH HIGH HIGH

MEDIUM HIGH HIGH

MEDIUM MEDIUM HIGH HIGH HIGH LOW LCW

MEDIUM LOW LOW LOW HIGH LOW HIGH

MEDIUM HIGH HIGH HIGH HIGH HIGH

Nm3/min/t 0.17-0.25 0.08-0.17 0-0.08

Ore Type

A A C F C C A B C D E E B A C C D A B A B C A A D A A

Coal Type

D D D E A D C A A B B B A A A B B A A A B A B B B B B

Ore Feed Method

Oxy. Lance Oxy.Lance Oxy.Lance Oxy.Lance Oxy.Lance Oxy. Lance Oxy.Lance Gravity Oxy.Lance Gravity Gravity Gravity Gravity

Oxy.Lance Oxy.Lance Oxy.Lance Gravity

Oxy.Lance Gravity Oxy.Lance Gravity

Oxy.Lance Aux.Lance Aux.Lance Gravity

Aux.Lance Aux.Lance

Coal Feed Method

> >

Oxy.Lance Oxy.Lance Oxy.Lance Oxy.Lance Oxy.Lance Oxy.Lance Oxy.Lance Oxy. Lance Oxy.Lance Aux.Lance Oxy.Lance Oxy.Lance Oxy.Lance Oxy.Lance Oxy.Lance Oxy.Lance Aux.Lance Oxy.Lance Oxy.Lance Oxy.Lance Aux.Lance Oxy.Lance Aux.Lance Aux.Lance Aux.Lance Aux.Lance Aux.Lance

TABLE 2

COMPOSITION OF RAW MATERIALS

ORE

A S.A.Fines B S.A.Briqu C Greek Fines D S.A.Briqu E Greek Briqu F S.A.Fines

FeT %

19.25 16.15 10.34 17.05 10.12 17.50

Cr 0 1 3

43.95 40.80 46.64 38.60 47.24 39.56

SiO %2

1.20 6.85 8.20 6.90 7.86 5.80

MgO %

10.00 15.10 18.60 15.10 17.80 14.60

CaO %

0.11 0.55 0.35 0.55 0.71 0.42

MnO %

0.21 0.01 0.18 0.23 0.16 0.22

Al O 1 3

15.70 13.95 7.87 15.20 7.96 15.20

S %

0.01 0.03 0.01 0.01 0.01 0.01

C %

0.00 0.75 0.40 0.00 0.00 0.11

00 <n

COAL

A Anthracite B Anthracite C Medium Vol D Anthracite E Medium Vol

Moisture %

1.70 0.31 1.36 1.60 1.60

Volatile %

9.88 4.31 18.00 8.28 17.90

Ash %

3.90 11.13 5.13 4.65 5.20

Fixed C %

84.52 84.24 75.51 85.47 77.40

C %

87.37 88.54 84.92 87.32 86.30

H %

3.70 3.57 4.24 3.60 3.60

S %

0.97 0.90 0.87 0.89 0.70

N %

0.92

1.25

C.V. MJAg 34.68 32.48 34.36 34.60 33.92

A B C

FLUXES

Gravel Lime Lime

FeT %

1.56

CaO %

19.58 >95 >95

SiO 2

%

50.90

MgO %

2.78

Al O 2 3 %

2.70

SIZE RANGE mm 5-15 40 15

TABLE 3

RAW MATERIAL PARTICLE SIZE ANALYSIS

FINE ORES

A C F

+1.7mm

%

0.00 6.24 0.00

—1.7mm +500//

%

5.21 37.82 18.47

-500// +300//

%

47.73 17.61 40.64

-300// +250//

%

10.53 4.40 3.47

-250// +212//

%

15.35 5.66 9.27

-212// +150//

%

12.63 7.51

10.11

-150// +106//

%

6.15 8.21 6.10

-106// +53//

%

2.26 6.94

10.90

-53//

%

0.13 5.60 1.02

BRIQUETTE ORES

B D E

Nominal Size mm

40 * 25 * 20 40 * 25 * 20 40 * 25 * 20

COALS

A B C D E

+2mm

%

6.00 7.00

1.33

—2mm +lmm

%

27.50 40.60 25.20 11.36 12.10

—1mm +500//

%

24.60 23.40 25.50 24.66 24.20

-500// +250//

%

17.20 12.00 15.70 22.00 25.70

-250// +125//

%

11.40 6.40

17.80 16.66

8.20

-125// +75//

%

9.90 3.80

13.30 10.00 9.90

-75// +38//

%

1.65 4.80 1.70

13.00 19

-38//

%

1.65 2.00 1.60 0.99

.90

TABLE 4

SUMMARY OF MATERIALS INPUTS AND OUTPUTS

00

HEAT NUMBER

104 103 105 107 87 106 102 82 90 99 100 101 80 76 89 91 98 79 81 77 96 88 92 93 97 94 95

INITIAL HOT METAL kg 2700 2600 2700 2700 2735 2800 2625 2575 2685 2435 2805 2720 3005 3300 2660 2425 2305 3125 2825 2400 2670 2635 2800 2640 2765 2805 2820

CHARGE E.A.F. SLAG kg 100 100 100 100 45 0 140 145 100 0 0 85 0 0 140 20 0 0 0 0 95 140 0 0 0 0 0

IN-BLOW ADDITIONS PROCESS TIME mins 176.5 160.7 198.1 113.5 63.1 139.1 123.3 74.3 100.3 100.8 102.9 182.3 53.9 100.5 77.9 61.7 66.8 35.4 52.0 22.0 54.5 29.2 43.3 39.1 73.1 40.2 36.3

OXYGEN Nm3

1397 1290 1588 904 544 1111 1005 571 829 812 826 1435 433 857 676 527 662 302 447 144 555 258 486 414 590 416 374

COAL kg 1953 1810 2383 1180 1087 1694 1213 755 948 960 1050 1785 576 1007 822 755 908 364 526 169 637 270 476 456 954 443 427

CHROME ORE kg 728 589 927 478 237 749 595 315 420 556 652 831 315 448 357 348 514 69 273 33 420 84 119 162 706 115 194

LIME kg 215 180 295 140 112 200 165 133 45 158 174 422 60 157 45 45 82 100 60 78 117 45 154 116 120 168 155

GRAVEL kg 0 0 10 0 31 40 0 0 54 0 0 390 36 72 107 87 50 20 58 5 0 8 89 60 0 60 70

END OF BLOW OUTPUTS METAL kg 2350 2860 2645 2510 2555 2285 2130 2565 2470 2300 2700 2640 2507 2774 2606 2220 1790 2747 2498 2300 2600 2540 2820 2645 2800 2355 2603

SLAG kg 990 890 1075 1030 460 550 785 525 795 410 600 1785 691 110 820 586 550 245 624 -390 375 590 390 570 390 455

FUME kg 300 288 250 162 304 467 379 135 245 320 210 426 295 335 67 9 199 210 113 80 171 0 — — 216 --

TABLE 5

CHROMIUM MASS BALANCES

ui ID

HEAT NUMBER

104 103 105 107 87 106 102 82 90 99 100 101 80 76 89 91 98 79 81 96 88 92 93 97 94 95

CHROMIUM INPUTS INITIAL METAL kg Cr 425.3 0.5 1.6 1.2 1.0 9.0 7.6 2.3 0.8 0.9 0.8 0.7 474.6 1.8 0.9 1.2 84.6 514.0 453.3 0.9 0.8 1.3 43.6 0.8 0.7 10.2

INITIAL SLAG kg Cr 0.5 0.4 0.6 0.6 0.1 0.0 0.8 9.8 0.2 0.0 0.0 0.2 11.7 0.0 0.4 0.3 0.0 1.9 6.0 0.1 0.9 0.0 0.0 0.0 0.0 0.0

ORE ADDITION kg Cr 218.9 177.1 304.3 128.5 75.6 245.9 186.0 87.9 137.9 146.9 210.8 266.7 87.9 134.8 113.9 112.6 135.8 20.7 76.2 114.8 26.8 38.1 48.7 186.5 34.6 58.3

TOTAL INPUTS kg Cr 644.8 178.0 306.6 130.4 76.7 254.9 194.6 100.0 138.9 147.7 211.6 267.8 574.2 136.6 115.2 114.1 220.4 536.6 535.5 115.9 28.5 39.4 92.3 187.3 35.3 68.5

CHROMIUM OUTPUTS METAL kg Cr 476.2 210.5 263.4 112.8 51.6 112.0 48.1 71.6 61.8 103.3 102.3 49.9 326.4 38.7 45.1 44.4 58.7 421.7 386.8 54.6 17.4 35.3 46.8 47.9 11.4 15.9

SLAG kg Cr 28.5 10.8 16.7 27.3 20.9 67.0 137.8 34.9 73.7 39.9 71.4 202.1 74.9 12.3 59.4 99.9 81.2 97.3 105.0 31.3 16.3 33.2 29.6 79.8 18.8 30.2

FUME kg Cr 28.6 6.8 14.1 3.2 2.0 12.1 5.7 1.5 2.2 3.2 7.5 6.1 18.3 0.7 0.5 0.4 3.5 17.7 8.2 1.4 0.5 0.0 0.0 1.9 0.0 0.0

TOTAL OUTPUTS kg Cr 533.2 228.1 294.3 143.3 74.5 194.0 191.6 108.0 137.7 146.3 181.2 258.2 419.6 51.7 105.0 144.7 143.4 536.6 500.0 87.3 34.2 68.5 76.4 129.6 30.2 46.1

CALCULATED YIELDS LOSS(-) GAIN(+) kg Cr -111.6 +50.1 -12.3 +12.9 -2.2 -63.8 -3.1 +8.0 -1.2 -1.4 -30.4 -9.6 -154.6 -84.9 -10.2 +30.6 -77.0 0.0 -35.5 -28.5 +5.7 +29.1 -15.9 -57.7 -5.1 -22.4

MIN %

73.8 92.3 85.9 78.7 67.3 43.9 24.7 66.3 44.5 69.9 48.3 18.6 56.8 28.3 39.1 30.7 26.6 78.6 72.2 47.2 50.9 51.5 50.7 25.6 32.3 23.2

MAX %

89.3 100.0 89.5 86.5 69.3 58.6 25.1 71.6 44.9 70.6 56.4 19.3 77.8 74.9 43.0 38.9 40.9 78.6 77.4 62.6 61.1 89.6 61.3 37.0 37.7 34.5

TABLE 6

SPECIFIC RAW MATERIAL CONSUMPTIONS

Specific Raw Material Consumption

HOT METAL PROCESS TIME

COAL ORE

OXYGEN LIME

CARRIER GAS PRODUCTS

METAL

GAS

SLAG

kg minutes kg/min/t kg/min/t Nm3/min/t kg/min/t Nm3/min/t

kg %Cr %C Nm3/min/t ICO %C02 %H2 %N2 %H20 %A kg/min/t Basicity %Cr203 %FeO

Calculated

1000 360

4.091 2.159 3.295 0.595 1.442

1183 18.6

5 9.76 60.08 7.50 16.91 14.83 0.67

-

1.55 2.0 3.7 3.3

Experimen

Whole Expt 2800 198.1 4.290 1.670 2.860 0.530 1.340

2645 9.96 5.85 10.86 56 7 17 7 — 13 1.60

2 6.9 3.5

tal Data

Best

4.107 2.500 2.975 0.530 1.340

40

TABLE 7 MATERIALS BALANCE TO PRODUCE 100 TONNES OF CHROME IRON

HOT METAL PROCESS TIME

COAL ORE

OXYGEN LIME

CARRIER GAS PRODUCTS

METAL

GAS

NCV SLAG

tonnes minutes tonnes tonnes tonnes tonnes tonnes

tonnes %Cr %C Nm3 %CO %C02 %H2 %N2 %H20 MJ/Nm3 tonnes Basicity %Cr203 %FeO

84.5 360 124.5 65.7 143.2 18.1 54.8

100 18.6 5

297008 57.24 7.15 17.40 17.64 0.57 9.20 47.2 2.0 3.7 3.3

41

TABLE 8

RAW MATERIAL CONSUMPTIONS IN STAGE ONE DECARBURISATION

CASE 1 NICKEL OXIDE ADDITION

RAW MATERIALS CHROME-IRON tonnes

NiO tonnes DOLOMET tonnes SCRAP tonnes

OXYGEN tonnes NITROGEN tonnes TIME mins

100.0 9.9 3.0 9.6 4.6 0.5

15.8 PRODUCTS

STEEL tonnes CO Nm3 C02 Nm3

115.0 7750 861

Temperature Degrees C 1600.00 25.00 25.00 25.00 25.00 25.00

Degrees C 1690.00 1645.00 1645.00

Carbon %

5.00

0.05

%C 0.40

Chromium %

18.70

18.00

%Cr 18.00

Nickel %

0.00 78.58 8.00

%Ni 8.00

CASE 2 NICKEL METAL ADDITION

RAW MATERIALS CHROME-IRON tonnes NICKEL tonnes DOLOMET tonnes SCRAP tonnes

OXYGEN tonnes NITROGEN tonnes TIKE mins

100.0 7.8 3.0

28.6 8.0 0.9 15.8

PRODUCTS STEEL tonnes CO Nn»3 002 N»3

130.0 7750 861

Temperature Degrees C 1600.00 25.00 25.00 25.00 25.00 25.00

Degrees C 1690.00 1645.00 1645.00

Carbon %

5.00

0.05

%C 0.30

Chromium %

18.70

18.00

%Cr 18.00

Nickel %

0.00 100.00 8.00

%Ni 8.00

42

TABLE 9

RAW MATERIALS COSTS SMELTING REDUCTION ROUTE

Smelting Reduction Material

Hot Metal Coal

Chromite Ore Oxygen Lime

Carrier Gas Total

Products Chrome-Iron Gas GJ Net Cost

Consumption tonnes 84.50 124.50 65.70 143.20 18.10 54.80

100.00 2732

Unit Cost £/tonne 90.00 44.10 51.17 21.89 5.31 7.80

(1.50)

Cost £

7605.00 5490.45 3361.87 3134.65 96.11 427.44 20115.52

(4098.71) 16016.81

Stage One Decarburisation Material

Chrome-Iron Nickel Oxide

Dolomet S.S. Scrap Oxygen Nitrogen Total

Products Crude Steel Gas GJ Net Cost

Cost/tonne ci

Consumption tonnes 100.00 9.90 3.00 9.60 4.50 0.50

115.00 98

rude steel

Unit Cost £/tonne 160.17 3493.27 39.74 370.00 21.89 7.80

(1.50)

471.56

Cost £

16016.81 34583.37 119.22 3552.00 100.69 3.90

54376.00

(146.81) 54229.19

43

Aim Slag Composition

20 30 40 Weight Percent MgO

50 — ►

60 70

FIG. 1

LIQUIDUS SURFACE OF THE CaO-MgO-.Si02-AlgOa. SYSTEM AT THE 20% A12Q3 PLANE SHOWING THE AIM SLAG COMPOSITION

44

<Fe,Cr>3C

20 40 60 60 CHROMIUM, WEIGHT PERCENT

FIG. 2

LIQUIDUS SURFACE IRON-CHROME-CARBON SYSTEM

45

Powdered Coal and Ore

Gas k Oxygen > Offtake \

Metal Tapping,

Sand Seal Between Vessel and Hood

Slag Tapping

Stirr ing Gas

FIG. 3

MAIN FEATURES OF PILOT PLANT

46

» ♦ 0 20 40 60 80 100 120 140 ISO ISO 200

20.

10

10, 20 40 60 80 100 120 140 ISO 180 200

^£ j >-

0 20 40 80 100 120 140 160 180 200

Procttt Tint <nln>

FIG. 4(a)

C.S .M. HEAT 103 - PROCESS SUMMARY

47

10.0

10.0,

S 3-0

20 40 CO 80 100 120 140 160 180 200

20 40 SO 60 100 1ZO 140 ISO ISO 200

9

« • m

3 . 0 .

2.5

2.0

I.S

1.0

0.5

0.0

Sl«9 Bot lc l t j • <C«0^0>/<SI02»A1203>

20 40 60 80 100 120 H O 160 160 200

Pr*>c«tt Tint <nln>

FIG. 4 (b )

C . S . M . HEAT 103 - PROCESS SUMMARY

48

3 o

CD

22

20

18

16

14

0 20 40 60 80 too 120 140 160 180 2C

i «>4 i «

160 1»0 200

20 40 60 80 100 120 140 160 180 200

•> ._ O 20 40 10* 80 100 120 140 160 180 200

^n 0 20 40 80 100 120 140 160 180 200

Proctt* T i m <nln>

FIG. 5(a)

C . S . M . HEAT 104 - PROCESS SUMMARY

49

10.0,

?.s

3.0

2.5

20 40 60 M 100 120 140 ISO 180 200

10.0

ft 5 5.0

80 100 120 140 ICO ISO 200

3 . 0 , Slog Battel tg s <CrfM1gO>/<ni203»SI02>

FIG. 5(b)

C . S . M . HEAT 104 - PROCESS SUMMARY

50

so. 20 40 GO 80 100 120 HO ISO 180 200

S 20.

10

ill

0 20 40

0 20 40 60 80 100 120 140 ISO 180 200

80 100 120 140 160 180 200

Proc«tt Ttn« <nln>

FIG. 6(a)

C.S.M. HEAT 105 - PROCESS SUMMARY

51

80 100 120 140 ISO 180 200

9 a

1 0 . 0 .

7.S

2.5

3 . 0 , Slog Battel Cy • <CaO«na0>/<f)l203*StO2>

2.S ■ ^ s

1.5

1.0

0.5

0.0 £0 <0 60 100 120 M O 160 190 200

Pr«««tt Tln« <n|n>

FIG. 6(b)

C.S.M. HEAT 105 - PROCESS SUMMARY

52

1 oP

>> s-l CD > O O CD

1/ '"I

Bath Carbon (%)

FIG. 7

MEAN BATH CARBON vs CHROME RECOVERY C.S.M. MASS BALANCES

53

0 10 20 30 40 SO SO 70 90 90 100 110 120 130 140 ISO

0 10 20 30 40 SO SO 70 80 90 100 110 120 130 140 ISO

- » t 0 ' 10 20 30 40 SO SO 70 80 90 100 110 120 130 140 ISO

HO 0 10 20 30 40 SO SO 70 80 90 100 110 120 130 140 ISO

20

10

20 0 10 20 30 40 50 60 70 90 100 110 120 130 140 ISO

in 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Procttt Tin* <nln>

FIG. 8(a)

C.S.M. HEAT 102 - PROCESS SUMMARY

54

2 . 0 , 0 10 20 30 «0 SO SO 70 80 30 tOO 110 120 130 140 ISO

SI09 B o t i e l t y s <CoO«MgOV<$l02»m203>

10 20 30 «0 SO 60 70 80 SO 100 110 120 130 HO ISO

Pro«««t T I I M <nln>

FIG. 8 (b )

C . S . M . HEAT 102 - PROCESS SUMMARY

55

o\°

U

> o V CD

100

" 80

E0

40

20

,.''' -l-""""l

,,.''' I „,''

, , ■

HIGH BRTH CRRBON EXPERIMENTS

Bath Temperature (°C)

FIG. 9

MEAN BATH TEMPERATURE vs CHROME RECOVERY C.S.M. MASS BALANCES

56

100

r I

80

SO • v

>-a. > o <_) UJ

40 I T

1.3 SLAG BASICITY </i)

2.3

FIG. 10

SLAG BASICITY vs CHROME RECOVERY

57

-?*%

t (102 x )

FIG. 11

MICROGRAPH OF A REACTING GRAIN FROM EXPERIMENT 105

58

Ur Fe

(800 x ) ^ .<-«^ Aw

FIG. 12 ELEMENT DISTRIBUTION OF A REACTING GRAIN FROM EXPERIMENT 105

59

O

100 t

90 .

80

70

H 60 > | 50 .,

30

20

10

_ CO _ C02 . . N2

02 ._ H2

' / \/

.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 Process Tine in Minutes

60.00

FIG. 13

DUCT GAS ANALYSIS FROM HEAT C . S . M . 81

10000,

9000

8000

m 7000

6000

5000,

<r

IS i 3 £ 4000

3

3000.

2000

1000

^^V^^ YYW^J

10 15 20 25 30 35 40 45 50 55

PROCESS TIME IN MINUTES

60

FIG. 14 C.S.M. HEAT 81 WASTE GAS VOLUME FLOW RATE

61

2 2 .

20

18

16

2 ««

I ,

CONSTANT ORE INPUT

+ „ EXPERIMENT 103 + EXPERIMENT 104

10

200 290 300 390 400 490 900

PROCESS TIME IN MINUTES

FIG. 15

SMELTING REDUCTION OF CHROME ORES COMPARISON BETWEEN EXPERIMENTAL RESULTS

AND CALCULATED CHROMIUM LEVELS

62

Electric Melting

VWY

Blast Furnace Hot Metal

Desiliconise Dephosphorise

Gas Collection

Chromium Enrichment

FIG. 16 POTENTIAL PROCESS ROUTES

Decarburisation and Refining

APPENDIX

TECHNICAL ANNEX

OBJECTIVE AND STRUCTURE OF THE RESEARCH The Production of Stainless Steels

Through Smelting Reduction of Chrome Ores Using Coal and Oxygen

OBJECTIVE

The objective of the project is to produce liquid ferrochrome by smelting-reduction of chrome ores using injected coal and oxygen such that it can be used directly for the production of stainless steel, thus developing a through-route from chrome ore to stainless steel without the need for expensive electrical energy.

The work will be based on pilot scale rectors operating in both batch and continuous mode, together with computing facilities for process modelling and data evaluation.

METHOD

The main elements of the research programme are:-

1. To carry out pilot scale studies using a coal-based continuously operating pilot plant to determine the optimum chrome ore, coal, oxygen input rates to achieve steady state operating conditions of metal composition (C, Fe, Cr), temperature and low levels of chromium oxide in the slag, together with an output gas of consistent quality and production rate.

2. To simulate the effect of using the off-gas in-house to accomplish some preliminary pre­reduction of the chrome ore by adjustment to the raw materials feed mixture and input rates. To establish the minimum quantity of coal required by the process to achieve an optimum yield and energy requirement.

3. To produce stainless steel directly from ferrochrome produced from inputs of chrome ore, coal and oxygen in a converter adapted for oxygen:argon mixtures.

4. To develop a charge balance and process rate model of the coal-based, through-route to stainless steel process using the results obtained from the pilot scale studies.

5. To relate the results of the pilot scale study to commercial operation and to compare the energy requirements between coal-based and conventional through-routes for stainless steel production.

6. To evaluate the economics of the optimum coal-based process route.

7. The results of the research will be the subject of a publication in the "Steel Research" series.

8. The research described above will be placed in the area covered by the Executive Committee Bl - Blast furnace technology and direct reduction.

64

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European Commission

EUR 13956 — Reduction of iron ores The production of stainless steel through smelting reduction of chrome ores using coal and oxygen

C. Treadgold

Luxembourg: Office for Official Publications of the European Communities

1997 — 64 pp. — 21.0 x 29.7 cm

Technical steel research series

ISBN 92-828-1483-1

Price (excluding VAT) in Luxembourg: ECU 11.50

The objective of the project is to produce liquid ferrochrome by smelting reduction of chrome ores using injected coal and oxygen such that it can be used directly for the production of stainless steel, thus developing a through-route from chrome ore to stainless steel without the need for expensive electrical energy.

The work will be based on pilot scale rectors operating in both batch and continuous mode, together with computing facilities for process modelling and data evaluation.

The main elements of the research programme are:

1. To carry out pilot scale studies using a coal-based continuously opera­ting pilot plant to determine the optimum chrome ore, coal, oxygen input rates to achieve steady state operating conditions of metal composition (C, Fe, Cr), temperature and low levels of chromium oxide in the slag, together with an output gas of consistent quality and pro­duction rate.

2. To simulate the effect of using the off-gas in-house to accomplish some preliminary pre-reduction of the chrome ore by adjustment to the raw materials feed mixture and input rates. To establish the minimum quantity of coal required by the process to achieve an optimum yield and energy requirement.

3. To produce stainless steel directly from ferrochrome produced from inputs of chrome ore, coal and oxygen in a converter adapted for oxygen-argon mixtures.

4. To develop a charge balance and process rate model of the coal-based, through-route to stainless steel process using the results obtained from the pilot scale studies.

5. To relate the results of the pilot scale study to commercial operation and to compare the energy requirements between coal-based and conven­tional through-routes for stainless steel production.

6. To evaluate the economics of the optimum coal-based process route.

7. The results of the research will be the subject of a publication in the 'Steel research' series.

8. The research described above will be placed in the area covered by the Executive Committee B1 'Blast furnace technology and direct reduction'.

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