Innovation in Electric Arc Furnaces: Scientific Basis for Selection

Yuri N. Toulouevski and Ilyaz Y. ZinurovInnovation in Electric Arc Furnaces2nd ed. 2013Scientific Basis for Selection10.1007/978-3-642-36273-6© Springer-Verlag Berlin Heidelberg 2013

Yuri N. Toulouevski and Ilyaz Y. Zinurov

Innovation in Electric Arc FurnacesScientific Basis for Selection

The Second Edition

Revised and Supplemented

Yuri N. Toulouevski

Holland Landing, ON, Canada

Ilyaz Y. Zinurov

Chelyabinsk, Russia

ISBN 978-3-642-36272-9e-ISBN 978-3-642-36273-6

Springer Heidelberg New York Dordrecht London

Library of Congress Control Number: 2013933295

© Springer-Verlag Berlin Heidelberg 2010, 2013

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Preface to the Second Edition

The efforts of the developers of innovations for EAFs have intensified significantly in the last several years. This was caused by tightening of economic and environmental requirements to steelmaking. Development of new processes and new types of the furnaces was a way to meet this challenge. 400 ton capacity furnaces with the 240–300 MVA transformers have been developed and implemented. Productivity of these furnaces exceeds 360 t/h. Even on 120-t EAFs productivity level has reached over 200 t/h. A new promising process namely continuous melting of scrap in liquid metal has been implemented not only in the conveyor furnaces (Consteel furnaces), but in the shaft furnaces as well. New methods of scrap charging in the shaft furnaces have been developed which have great technological and environmental advantages. Consteel furnaces started using not only the heat of off-gases for scrap preheating but the burners as well.

The absence of the unbiased comparative analysis of new furnaces and technological processes makes difficult choosing between them, since the advantages of innovations are advertised, whereas the deficiencies are concealed. Potential users of the abovementioned innovations really need such analysis. The authors’ works regarding these important issues were not yet finished by the time of the first edition of this book. That is why the second edition became a necessity.

The promising process of continuous melting of scrap in liquid metal, its advantages, and limiting factors are reviewed in detail in the Chaps. 1 , 6 , and 7 of the second edition. The performance indices of the conveyor and shaft furnaces using this process as well as heating of scrap by off-gases are compared to those of the modern EAFs operating without scrap preheating. The root causes of low energy effectiveness of heating of scrap by off-gases are discussed, and rationale for turning away from such heating is given.

Developed by the authors design concepts of fuel-arc furnaces (FAF) with continuous scrap melting and its preheating up to 800 °C by powerful oxy-gas burners either on a conveyor or in a shaft are suggested. In FAF, electric energy consumption is reduced to 200 kWh/t with gas flow rate of 20 m ³ /t. Such furnaces can successfully compete with the most advanced modern EAFs and replace them. In the new edition, the latest innovations and other up-to-date information are added to the Chaps. 1 , 10 , and 14 , etc.

The authors express their gratitude to the readers who gave their feedback. All their observations and considerations are taken into account in the second edition of the book. The authors thank Ch. Baumann and I. Falkovich for their help and cooperation. Our special gratitude goes to G. Toulouevskaia for the great work she did for preparing of this publication.

The Authors

Preface to the First Edition

Selection of innovations for each plant as well as selection of directions of further development is one of the crucial problems both for the developers and for the producers of steel in EAF. Ineffective selection leads to heavy financial losses and waste of time. In practice, this happens quite frequently.

The main objective of this book is to help the readers avoid mistakes in selecting innovations and facilitate successful implementation of the selected innovations. The entire content of the book is aimed at achieving this objective. This book contains the critical analysis of the main issues related to the most widespread innovations in EAF. The simplified methods of calculations are used for quantitative assessment of innovations. These methods are explained by numerous examples. Considerable attention is given to the new directions of development which the authors consider to be the most promising.

In the process of writing of the book, its content was discussed with many specialists working at metallurgical plants and for scientific research and development organizations. The authors express deep gratitude for their valuable observations and considerations.

A number of the important issues covered in the book are debatable. The authors would like to thank in advance those readers who will consider it possible to take the time to share their observations. Their input will be really appreciated and taken into account in further work.

Our heartfelt thanks go to G. Toulouevskaia for her extensive work on preparation of the manuscript for publication.

Yuri N. Toulouevski

Ontaria, Canada

Introduction

Electric Arc Furnaces (EAF) are being greatly improved at a fast pace. Only 20–30 years ago, today’s EAF performance would be impossible to imagine. Owing to the impressive number of innovations, the tap-to-tap time has been shortened to 30–35 min for the best 100–130 t furnaces operating with scrap. Hourly productivity increased by six times, from 40 up to 240 t/h. Electrical energy consumption got reduced approximately 1.8 times, from 630 to 340 kWh/t. Electrical energy share in overall energy consumption per heat dropped to 50 %. Electrode consumption was reduced by about six times, Fig. 1 . One might expect such performances should be normal for most of steelmaking shops in the immediate future.

The technological function of EAF was drastically changed. All the technological processes providing both steel qualities required and its special properties have been moved out the furnaces to secondary ladle metallurgy equipment. ¹ The necessary increase in furnace productivity could not be achieved without this revolutionary change in EAF steelmaking. The main technological processes in the modern furnaces are melting of solid charge materials and heating of liquid bath. It is precisely these substantial thermal-energy processes that now define furnace productivity. To get these processes going, it is necessary to obtain heat from other kinds of energy (electrical or chemical) and transfer it to zones of solid charge or liquid bath. This is why the electric arc furnaces themselves and the processes in them are reviewed in this book mainly from the unified thermal-energy point of view.

These furnaces turned to be very flexible in terms of charge materials selection. They could readily accommodate to melting in various combinations steel scrap, pig iron and hot metal, and reduced iron as pellets or briquettes. In the majority of furnaces, metal charge consists of scrap with small additions of pig iron. Traditionally, scrap is charged into the furnace from above as a single charge or in two-three portions. Only at the so-called Consteel furnaces and at certain shaft furnaces, scrap is practically continuously charged by means of a conveyer or a hydraulic pusher via a furnace sidewall door. The wide variety of innovations being offered by the developers for each particular case corresponds to the various furnace operation conditions.

Changes in heat techniques, furnace designs, and equipment are taking place at a fast pace. Every year, new technical solutions are offered and widely advertised. Steel manufacturers have difficulty navigating through the flood of innovations. Under steep competition, advertisement information is somewhat biased and incomplete. This makes selection of innovations for solving particular problems even harder. It is not easy to decide which information is trustworthy enough. But it is much more difficult to decide what to select: an innovation which was already proved by practice or it is better to take a risk of the first realization which in the case of success promises maximum economical effect. Frequently, the cost of new technologies exerts the decisive influence on this selection. Certainly, the price is one of key criteria. But the other not less important factors such as, for instance, a new equipment reliability have to be taken into consideration. Therefore, when being based for the most part on a price, a serious error can be made.

What could help to carry out unbiased analysis of innovations and select those which could yield the best results for particular circumstances of a given plant? First of all, comprehensive understanding of mechanisms and basic laws defining the main processes of the EAF heat is required. The modern concepts of these processes are presented in numerous magazine papers and reports from technical conferences which are held worldwide on a regular basis. For a practical steelmaker, it is hard to get reliable general information necessary to solve specific practical problems. Meanwhile, the knowledge of general simplified yet correct in principle concepts is sufficient for decision making. These general concepts are currently commonly accepted. Without compromising scientific strictness, these principles are discussed in this book at the level easy to understand for the readers who do not have an adequate background in this field.

Data on effectiveness of any proposed innovation must not contradict proven principles of the processes of the heat. If such a contradiction takes place, a proposal should be excluded from further consideration. Regretfully, experience proves that innovations which do contradict to the basic principles are proposed rather frequently.

A typical case can be shown. Up to this point, various methods of bath oxygen blowing are proposed in order to provide carbon oxidation inside the bath not to carbon monoxide CO as it takes place in reality but to carbon dioxide CO 2 . If this would be possible, both heat amounts released in the bath and bath heating rate could be increased several times. However, according to the basics of physical chemistry of steelmaking processes, formation of CO 2 in presence of liquid iron is possible only in practically insignificant amounts. This basic principle was responsible for the failure of all attempts to oxidize carbon in steel bath to CO 2 . These attempts were repeatedly undertaken in the past in both open hearth furnaces and oxygen converters.

This example demonstrates that historical approach to the analysis of innovations proposed is very helpful. Such an approach is widely used in various chapters of this book. In certain cases, data, obtained not only in the modern steelmaking units but also in the obsolete open hearth furnaces, are used. When evaluating innovations for electric arc furnaces, the experience from open hearth furnaces as well as from converters proves to be highly useful. This is particularly relevant for the results of scientific and industrial studies of oxygen blowing in open hearth bath since the studies similar in scale, accuracy of experimental procedure, and resultant effectiveness have not been conducted in EAF.

Simplified calculations should be used for the preliminary comparative evaluation of innovations. Such calculations can be done manually by using regular calculators. Their accuracy is quite sufficient for the purpose pointed out. In many cases, the accuracy is not inferior to the accuracy of calculations which use complex methods of mathematical simulation. It can be explained due to the fact that often the input parameters for calculations are known quite approximately, and the accuracy of final results cannot exceed the accuracy of the input data regardless of calculation technique applied. In this regard, the mathematical calculations are similar to millstones: whatever you pour in that what you will get.

It should be emphasized, that carrying out even the very simple calculations greatly promotes comprehensive understanding of physical basics of processes and effects produced by various factors. Using off the shelf programs developed by means of the mathematical simulation of processes does not provide such possibilities. For the consumer, these programs are similar to a black box which does not reveal the mechanism of the process. The black box produces the final result but does not allow judging the conformity of the calculation to all the conditions of the specific case. Therefore, common off the shelf programs must be used with a great caution for evaluation of specific innovations.

When evaluating innovations which require heat balances of EAF, it is necessary to calculate thermal effects of exothermic reactions of oxidation of carbon, iron and its alloys. These thermal effects strongly depend on temperature of the initial substances and chemical reaction products. In a series of important cases, an effect of temperatures is not taken into account or it is not completely considered in the tables available to the readers. This leads to significant errors in calculations. In this book, an accurate and universal method, which is appropriate in all cases, is offered to determine influx of chemical heat. It is based on so-called method of full enthalpies and is very convenient for practical use.

Currently, most of innovations for EAF are aimed at the development of means and methods providing further intensification of processes of solid charge melting and liquid bath heating. Calculations in this field require knowledge of processes of heat transfer as well as hydro- and aerodynamics. To help readers mastering such calculations, several chapters containing required minimum of information in these fields of science are included in the book. This information is presented in a rudimentary form yet not compromising strict scientific meaning. Formulae for calculations are given in simplified form convenient for practical computing. Nevertheless, in doing so, the accuracy of calculations is maintained. Application of these formulae is illustrated by a large number of examples for analysis of innovations. Getting familiar with material in this book will allow the reader to perform required calculations on his own. In order to facilitate still further the performance of calculations, reference data needed for calculations are given in the book. This permits the readers to do away with the problem of searching for such data in various handbooks.

The book covers a wide variety of topics ranging from scientific concepts to state-of-the-art improvement practice of steelmaking in EAF. The book also contains new, progressive, in authors’ opinion, ideas on key issues regarding intensification of the heat such as scrap heating using high power oxy-fuel burners, deep bath blowing with oxygen and carbon using high-durable tuyeres, etc.

Significant attention is given to analysis of various directions of automation of the energy modes control of the heat. The descriptions of different automated control systems are drawn up by their developers according to the same principle and in essence differ only slightly from each other. Usually, the system functions are enumerated in detail. For example, the system controls the consumptions of electrical energy, oxygen, and fuel ensuring their savings and the increase in furnace productivity. But there is no information on how this is being done or on a specific algorithm (mechanism) of the system operation. Therefore, both estimation and selection of innovations in this field present great difficulties for metallurgists. The method for comparing the automation systems based on analysis of information used for controlling the heat is outlined in this book. This method provides a means for easy understanding of real and alleged advantages of a particular system as well as for making a justified decision.

The last chapter of the book deals with environment protection from gas and dust emissions of arc furnaces. A problem of reduction in energy gas evacuation costs is reviewed with consideration for current tendencies.

You can assess this book based on its contents. It is addressed to a wide range of EAF-steelmakers and all other metallurgists related to this industry. This range includes, among other, three categories of specialists: those who have to effectively use innovations in day-to-day practical work, those responsible for selection of innovations for their factories, and the developers of new processes and equipment for EAF. The book can also be used as a textbook for students of all levels studying metallurgy.

Fig. 1

Improvement in the 120-t EAF performances. 1 productivity, t/h, 2 electrical energy consumption, kWh/t, 3 electrode consumption, kg/t

Contents

1 Modern Steelmaking in Electric Arc Furnaces: History and Development 1

1.1 General Requirements to Steelmaking Units 1

1.1.1 Process Requirements 2

1.1.2 Economic Requirements 2

1.1.3 Environmental and Health and Safety Requirements 5

1.2 High-Power Furnaces: Issues of Power Engineering 7

1.2.1 Increasing Power of EAF Transformers 7

1.2.2 Specifics of Furnace Electrical Circuit 8

1.2.3 Optimum Electrical Mode of the Heat 11

1.2.4 Direct Current Furnaces 12

1.2.5 Problems of Energy Supply 13

1.3 The Most Important Energy and Technology Innovations 14

1.3.1 Intensive Use of Oxygen, Carbon and Chemical Heat 14

1.3.2 Foamed Slag Method 15

1.3.3 Furnace Operation with Hot Heel 18

1.3.4 Single Scrap Charging 18

1.3.5 Use of Hot Metal and Reduced Iron 19

1.3.6 Post-Combustion of CO Above the Bath 20

1.3.7 Increase in Capacity of Furnaces 21

1.3.8 Continuous Charging and Melting of Scrap in the Liquid Bath 22

References 24

2 Electric Arc Furnace as Thermoenergetical Unit 25

2.1 Thermal Performance of Furnace: Terminology and Designations 25

2.2 External and Internal Sources of Thermal Energy: Useful Heat 27

2.3 Factors Limiting the Power of External Sources 28

2.4 Key Role of Heat Transfer Processes 29

Reference 31

3 The Fundamental Laws and Calculating Formulae of Heat Transfer Processes 33

3.1 Three Ways of Heat Transfer: General Concepts 33

3.2 Conduction Heat Transfer 34

3.2.1 Fourier’s Law. Flat Uniform Wall. Electrical–Thermal Analogy 34

3.2.2 Coefficient of Thermal Conductivity 37

3.2.3 Multi-Layer Flat Wall 39

3.2.4 Contact Thermal Resistance 41

3.2.5 Uniform Cylindrical Wall 42

3.2.6 Multi-Layer Cylindrical Wall 43

3.2.7 Simplifying of Formulae for Calculation of Cylindrical Walls 44

3.2.8 Bodies of Complex Shape: Concept of Numerical Methods of Calculating Stationary and Non-Stationary Conduction Heat Transfer 45

3.3 Convective Heat Exchange 49

3.3.1 Newton’s Law: Coefficient of Heat Transfer α 49

3.3.2 Two Modes of Fluid Motion 50

3.3.3 Boundary Layer 50

3.3.4 Free (Natural) Convection 52

3.3.5 Convective Heat Transfer at Forced Motion 53

3.3.6 Heat Transfer Between Two Fluid Flows Through Dividing Wall; Heat Transfer Coefficient k 55

3.4 Heat Radiation and Radiant Heat Exchange 58

3.4.1 General Concepts 58

3.4.2 Stefan–Boltzmann Law; Radiation Density; Body Emissivity 59

3.4.3 Heat Radiation of Gases 62

3.4.4 Heat Exchange Between Parallel Surfaces in Transparent Medium: Effect of Screens 63

3.4.5 Heat Exchange Between the Body and Its Envelope: Transparent Medium 65

3.4.6 Heat Exchange Between the Emitting Gas and the Envelope 66

4 Energy (Heat) Balances of Furnace 67

4.1 General Concepts 67

4.2 Heat Balances of Different Zones of the Furnace 69

4.3 Example of Heat Balance in Modern Furnace 71

4.4 Analysis of Separate Items of Balance Equations 72

4.4.1 Output Items of Balance 72

4.4.2 Input Items of Balance 75

4.5 Chemical Energy Determination Methods 76

4.5.1 Utilization of Material Balance Data 76

4.5.2 About the So-Called Energy Equivalent of Oxygen 76

4.5.3 Calculation of Thermal Effects of Chemical Reactions by Method of Total Enthalpies 77

References 82

5 Energy Efficiency Criteria of EAFs 85

5.1 Preliminary Considerations 85

5.2 Common Energy Efficiency Coefficient of EAF and Its Deficiencies 87

5.3 Specific Coefficients η for Estimation of Energy Efficiency of Separate Energy Sources and EAF as a Whole 89

5.4 Determining Specific Coefficients η 92

5.4.1 Electrical Energy Efficiency Coefficient η EL 92

5.4.2 Fuel Energy Efficiency Coefficient of Oxy-Gas Burners η NG 93

5.4.3 Energy Efficiency Coefficient of Coke Charged Along with Scrap 94

5.4.4 Determining the Specific Coefficients η by the Method of Inverse Heat Balances 95

5.5 Tasks of Practical Uses of Specific Coefficients η 95

References 97

6 Preheating of Scrap by Offgases in Combination with Burners 99

6.1 Potentials and Limiting Factors 99

6.1.1 Expediency of Heating 99

6.1.2 Comparison of Consumptions of Useful Heat for Scrap Heating, Scrap Meltdown, and for Heating of Metal up to Tapping Temperature 100

6.1.3 Reduction in Electrical Energy Consumption with High-Temperature Heating of Scrap: Calculation of Potentials 101

6.1.4 Sample of Realization of High-Temperature Heating: Process BBC-Brusa 102

6.1.5 Specifics of Furnace Scrap Hampering Its Heating 103

6.2 Heating on Conveyor 105

6.2.1 Consteel Furnaces with Continuous Scrap Charging into the Bath 105

6.2.2 Comparison of Melting Rates, Productivities, and Electrical Energy Consumptions Between the Consteel Furnaces and EAFs 106

6.2.3 Scrap Preheating Temperature 109

6.3 Heating Scrap in a Large-Thickness Layer 111

6.3.1 Heat Transfer Processes 111

6.3.2 Heating Scrap in Baskets and Special Buckets 114

6.3.3 Twin-Shell Furnaces with Removal of Off-Gas Through the Second Bath 118

6.4 Heating Scrap in Shaft Furnaces 120

6.4.1 Shaft Furnaces with Fingers Retaining Scrap 120

6.4.2 Shaft Furnaces with Continuous Scrap Charging into the Liquid Bath by Pushers 122

6.5 From Utilizing Off-Gases to Scrap Preheating by Burners Only 126

References 127

7 Replacement of Electric Arcs with High Power Oxy-Gas Burners 129

7.1 Attempts for Complete Replacement 129

7.2 Potentialities of Existing Burners: Heat Transfer, Limiting Factors 131

7.3 High-Power Rotary Burners (HPR-Burners) 134

7.3.1 Fundamental Features 134

7.3.2 Slag Door Burners: Effectiveness of Flame-Direction Changes 134

7.3.3 Roof Burners 136

7.3.4 Oriel Burners 138

7.3.5 Sidewall Burners 140

7.4 Two-Stage Process of the Heat with Use of HPR Burners: Industrial Trials 143

7.4.1 General Energy Ratios 143

7.4.2 Process with a Door Burner in 6-ton Furnaces 145

7.4.3 Process with Roof Burners in 100-ton and 200-ton Furnaces 148

7.5 Fuel Arc Furnaces (FAFs) 151

7.5.1 FAF with Scrap Heating in a Furnace Freeboard 151

7.5.2 Conveyor FAFs with Continuous Scrap Charging into the Liquid Bath 153

7.5.3 Shaft FAFs with Continuous Scrap Charging by a Pusher 155

7.6 Economy of Replacement of Electrical Energy with Fuel 157

References 160

8 Basic Physical–Chemical Processes in Liquid Bath Blown with Oxygen: Process Mechanisms 161

8.1 Interaction of Oxygen Jets with the Bath: General Concepts 161

8.2 Oxidation of Carbon 163

8.3 Melting of Scrap 164

8.4 Heating of the Bath 166

9 Bath Stirring and Splashing During Oxygen Blowing 169

9.1 Stirring Intensity: Methods and Results of Measurement 169

9.2 Mechanisms of Bath Stirring 170

9.2.1 Stirring Through Circulation and Pulsation 170

9.2.2 Stirring by Oxygen Jets and CO Bubbles 171

9.3 Factors Limiting Intensity of Bath Oxygen Blowing in Electric Arc Furnaces 172

9.3.1 Iron Oxidation: Effect of Stirring 172

9.3.2 Bath Splashing 174

9.4 Oxygen Jets as a Key to Controlling Processes in the Bath 177

References 178

10 Jet Streams: Fundamental Laws and Calculation Formulae 179

10.1 Jet Momentum 179

10.2 Flooded Free Turbulent Jet: Formation Mechanism and Basic Principles 180

10.3 Subsonic Jets: Cylindrical and Tapered Nozzles 182

10.4 Supersonic Jets and Nozzles: Operation Modes 186

10.5 Simplified Formulae for Calculations of High-Velocity Oxygen Jets and Supersonic Nozzles 188

10.5.1 A Limiting Value of Jets’ Velocity 190

10.6 Long Range of Jets 191

Reference 191

11 Devices for Blowing of Oxygen and Carbon into the Bath 193

11.1 Blowing by Consumable Pipes Submerged into Melt and by Mobile Water-Cooled Tuyeres 193

11.1.1 Manually Operated Blowing Through Consumable Pipes 194

11.1.2 BSE Manipulator 194

11.1.3 Mobile Water-Cooled Tuyeres 196

11.2 Jet Modules: Design, Operating Modes, Reliability 199

11.2.1 Increase in Oxygen Jets Long Range: Coherent Jets 201

11.2.2 Effectiveness of Use of Oxygen, Carbon, and Natural Gas in the Modules 203

11.3 Blowing by Tuyeres Installed in the Bottom Lining 205

11.3.1 Converter-Type Non-Water-Cooled Tuyeres 205

11.3.2 Tuyeres Cooled by Evaporation of Atomized Water 207

11.3.3 Explosion-Proof Highly Durable Water-Cooled Tuyeres for Deep Blowing 209

References 214

12 Water-Cooled Furnace Elements 215

12.1 Preliminary Considerations 215

12.2 Thermal Performance of Elements: Basic Laws 215

12.3 Principles of Calculation and Design of Water-Cooled Elements 219

12.3.1 Determining of Heat Flux Rates 219

12.3.2 Minimum Necessary Water Flow Rate 221

12.3.3 Critical Zone of the Element 222

12.3.4 Temperature of Water-Cooled Surfaces 222

12.3.5 Temperature of External Surfaces 225

12.3.6 General Diagram of Element Calculation 226

12.3.7 Hydraulic Resistance of Elements 226

12.4 Examples of Calculation Analysis of Thermal Performance of Elements 229

12.4.1 Mobile Oxygen Tuyere 229

12.4.2 Elements with Pipes Cast into Copper Body and with Channels 231

12.4.3 Jet Cooling of the Elements 234

12.4.4 Oxygen Tuyere for Deep Blowing of the Bath 235

References 237

13 Principles of Automation of Heat Control 239

13.1 Preliminary Considerations 239

13.2 Automated Management Systems 239

13.2.1 Use of Accumulated Information: Static Control 239

13.2.2 Mathematical Simulation as Method of Control 240

13.2.3 Dynamic Control: Use of On-line Data 243

13.3 Rational Degree of Automation 249

References 250

14 Off-Gas Evacuation and Environmental Protection 251

14.1 Preliminary Considerations 251

14.2 Formation and Characteristics of Dust–Gas Emissions 251

14.2.1 Sources of Emissions 251

14.2.2 Primary and Secondary Emissions 252

14.2.3 Composition, Temperature, and Heat Content of Off-Gases 253

14.3 Capturing Emissions: Preparing Emissions for Cleaning in Bag Filters 255

14.3.1 General Description of the System 255

14.3.2 Problems of Toxic Emissions 256

14.3.3 A Simplified Method of Gas Parameters’ Calculation in the Direct Evacuation System 259

14.3.4 Energy Problems 268

14.4 Use of Air Curtains 270

References 275

Index277

Footnotes

1

These processes and equipment are not considered in the book.

Yuri N. Toulouevski and Ilyaz Y. ZinurovInnovation in Electric Arc Furnaces2nd ed. 2013Scientific Basis for Selection10.1007/978-3-642-36273-6_1© Springer-Verlag Berlin Heidelberg 2013

1. Modern Steelmaking in Electric Arc Furnaces: History and Development

Yuri N. Toulouevski¹   and Ilyaz Y. Zinurov²  

(1)

Oakridge Court 303-84, Holland Landing, ON, L9N 1R4, Canada

(2)

Engels Str. 77A, #20, Chelyabinsk, Russia, 454080

Yuri N. Toulouevski (Corresponding author)

Email: [email protected]

Ilyaz Y. Zinurov

Email: [email protected]

Abstract

Process, ecological, economic, health, and safety requirements in steelmaking units. The change in these requirements with developing of current steel production. A negative effect of existing contradictory practice is the forming in prices for scrap, iron, electrical energy, and natural gas on general technological progress in steelmaking. Increasе in productivity is the most important direction in EAF development. Due to numerous innovations implemented during the last decades, hourly furnace productivity has been increased by 6 times, electrical energy consumption has been decreased by approximately 1.8 times, and electrode consumption reduced by 6 times. The following innovations are analyzed in detail: increase in power of EAF transformers up to 1.0 – 1.5 MVA/t; implementing the secondary ladle metallurgy; intensive oxygen and carbon injection into a bath; slag foaming; furnace operation with hot hill; single scrap charging; using hot metal; continuous charging and melting of scrap in the liquid bath, etc. Electrical circuit specifics of modern EAFs, optimum electrical mode of the heat, DC furnaces, and problems of electrical energy supply are discussed as well.

1.1 General Requirements to Steelmaking Units

The structure of modern steelmaking has been formed gradually during the last 100 years. In this period, due to many different reasons, the requirements to steelmaking units have changed substantially. Some production methods have appeared and developed, while other ones have become noncompetitive and have been rejected. All these changes were interrelated and influenced each other. The understanding of electric steel production development and its prospects can not be complete if this process is studied separately setting aside the development of steelmaking in general. Therefore, it is necessary, even if briefly, to review the history of not only electric arc furnaces but also other steelmaking units competing with each other.

Steelmaking units should meet a number of requirements that could be classified into four groups in the following way:

1.

Process requirements ensure the necessity to produce various steel grades of required quality.

2.

Economic requirements call for reduction of manufacturing costs so as to increase profitability and competitiveness of products.

3.

Environmental requirements do not permit any excessive environment pollution, the level thereof being governed by state regulations.

4.

Health and safety requirements exclude the use of physically and psychologically straining labor which, at a certain stage of social development of society, becomes unacceptable for the population of a given country.

In any case, all innovations introduced in steelmaking have always been aimed at fulfilling some or all of the above mentioned requirements. However, the influence of these requirements has been changing greatly in the course of time.

1.1.1 Process Requirements

Up to the middle of the twentieth century, the most important changes in steelmaking were instigated by these very requirements. At the very beginning of the century, they led to development and wide spread of the electric arc furnaces (EAFs), since these units made it possible to easily achieve highest temperatures and ensured the best conditions for producing of high-quality alloyed steel grades and alloys. Previously, such metal could be produced by the crucible method only. Due to its inefficiency and too high requirements to the purity of raw materials, this method could not compete with the EAF process. A demand for special expensive steels and alloys with particular properties was quickly increasing. Electric arc furnace became the main supplier of such metals, though it was also used for production of relatively small quantities of common steel.

The process requirements were also a reason for replacement of acid and basic Bessemer converters with open-hearth furnaces. Due to increased nitrogen content, the quality of steel produced in the air-blast converters was greatly inferior to that of the open-hearth steel. As a result, the open-hearth method has become prevailing method of steel mass production, right up to the development of oxygen converters and even somewhat later.

The process requirements ceased to have a substantial effect on the relative competitiveness of basic steelmaking units when the ladle furnaces were introduced and became widespread as molten metal treatment units. At present, both oxygen-blown converters and EAFs usually produce semi-products of preset temperature and carbon concentration. This metal is treated to reaching the final chemical composition, refined by removing dissolved gases and non-metallic inclusions therein, and heated up to optimal temperature in ladle furnaces and other secondary metallurgy units.

Practically every steel grade can be produced by this way. The only obstacle encountered when producing some specific steel grades in EAFs is the contamination of scrap with copper, nickel, chrome and other residual contaminants which can not be removed in the course of processing of the finished steel. Permissible content of these contaminants is strictly limited in quality steel grades. This obstacle is overcome by means of more careful scrap preparation as well as by partial substitution of scrap with hot metal or products of direct iron reduction. Recently, such products are used in electric steelmaking rather widely.

1.1.2 Economic Requirements

The cost of scrap and ferroalloys amounts to approximately 70% of the general costs in EAFs operating on scrap. The so-called costs of operating constitute the rest 30%; the cost of electrical energy, fuel and electrodes account for about 40% of the latter. There are three possible ways of reducing the costs:

1.

By cutting down specific consumption of charge materials, energy-carriers, refractory materials etc. per ton of steel.

2.

By increasing output and thus reducing specific manufacturing costs, such as maintenance staff costs etc.

3.

By replacing expensive charge materials and energy-carriers with cheaper ones.

Innovations developed in the first direction are always justified as well as those in the second one. For more than half a century, the main direction of development of electric arc furnaces is increasing of their productivity. Almost all innovations, implemented in this period of time, were aimed at this problem. Without solving this problem the EAF could have never become the very steelmaking unit which along with oxygen converter is a determinant of world steelmaking.

Excluding the cost of metal charge the productivity is a parameter on which the entire economics of steelmaking process depends to the greatest degree. As a rule, when productivity is increased, manpower and maintenance costs are reduced, as well as costs of electrical energy, electrodes, fuel, refractories and other so-called costs of operating, including overall plant expenditures.

Electric arc furnaces are mostly intended to be installed at mini-mills where they determine productivity of the entire plant. Increasing output of mini-mills to one-two million of tons per year or even more had decisive effect on the maximum productivity level of EAF. It is most reasonable to equip steelmaking shops at such plants with one furnace, two at the maximum. Such organization of production allows minimizing manpower and operating costs in general.

If the shops are equipped with a number of furnaces then under conditions of extremely high pace of operation it is impossible to avoid some organizational delays. Any disruption of the preset production pace at one of the furnaces adversely affects other furnaces thus reducing significantly the shop productivity and that of the plant as a whole. Therefore, preference is given to the shops equipped with one furnace, even in the cases when required output exceeds 2.0–2.5 million ton per year.

Innovations developed in the third group are not always justified. Prices on materials and energy-carriers are subjected to rather abrupt fluctuations so that they are difficult forecast. In different countries, they can change dissimilarly and even in the opposite directions. That is why the innovations, determined only by a price difference, are associated with relatively high risks, especially when they are aimed for long-term and wide spread.

Let us discuss a number of examples. Scrap was substantially cheaper than hot metal for a long time nearly everywhere. Under such conditions, increasing amount of scrap re-melted in oxygen converters aiming at reducing hot metal consumption, could promote a significant increase in converter steel profitability. To achieve this various methods were developed to introduce additional heat into converters, such as scrap preheating by powerful oxy-fuel burners, introduction of coal and other carbon-containing additives into the charge, post-combustion of carbon monoxide evolved in the converter etc. Developing out and mastering these innovations was associated with significant difficulties. To overcome these difficulties long-term extensive industrial research accompanied by vast spending was conducted in a number of countries.

However, the interest in all these innovations was gradually declining as scrap price was increasing and approaching the price of hot metal. Therefore, replacing hot metal with scrap in converters was stopped. On the contrary, in the recent years, hot metal started to be used in EAFs in increasing amounts. This assured significant reduction of tap-to-tap time and electrical energy consumption, and also promoted production of such steel grades which require charge rather free from foreign contamination.

At present, a situation similar to that of replacing hot metal with scrap is developing with regard to innovations aiming at substituting electrical energy with the natural gas energy in EAFs. Just recently, this aim was justified by low cost of gas compared to electrical energy. At first glance, such price ratio could not change substantially, since a significant share of electrical energy is produced at thermal power plants using gas. However, in reality in most of countries, the price of natural gas was rising many times more quickly than the price of electrical energy. For example, in the USA the price of natural gas has increased by