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(1)AKADEMIA GÓRNICZO-HUTNICZA im. Stanisława Staszica w Krakowie. Faculty of Materials Science and Ceramics Department of Physical Chemistry and Modelling. Ph.D. Thesis. 3D pore structure and infiltration resistance of micropore carbon materials Jakub Stec Thesis supervisor: dr hab. inż. Robert Filipek, prof. AGH. Kraków 2020.

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(3) Acknowledgments I would like to express my gratitude toward all people without whom this dissertation would never exist. First and foremost, I would like to thank my supervisor, dr inż. hab. Robert Filipek, Prof. AGH-UST, for his patience and constant support. His mentorship was priceless in providing knowledge and experience required in my research. I am extremely grateful that I had the opportunity to start my scientific work under his guidance. Special thanks to Janusz Tomala for all of his lectures and discussions about carbon and graphite materials, which convinced me to get into this topic. Not only he extended my knowledge, but what is even more important, he created the possibility for our collaboration during my master and doctoral thesis. I would also like to thank Frank Hiltmann for his guidance, project supervision and valuable discussions. Many thanks go to all colleagues from Tokai COBEX: Mariusz Minkina, Sabina Czech and Piotr Kubica for their help with sample preparation, mercury intrusion porosimetry and hot metal penetration tests. Last but not the least, I would like to thank dr hab. inż. Jacek Tarasiuk, Prof. AGH-UST, for his enormous work in the field of X-ray computed tomography measurements of micropore carbon materials. Also, I would like to thank Dr.-Ing. Jürgen Gluch for his invaluable help at the beginning of my work on 3D microstructure analysis. Finally, I would like to thank my wife, family and colleagues from the Faculty of Materials Science and Ceramics AGH for their support..

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(5) Contents Preface ........................................................................................................................................................ 9 List of abbreviations....................................................................................................................................13 1. Blast Furnace ......................................................................................................................................17 1.1. 1.2. 1.3. 1.4. 2.. Mechanisms of blast furnace hearth degradation ................................................................................25 2.1. 2.2. 2.3. 2.4. 2.5. 2.6. 2.7.. 3.. Alkali Attack Test .........................................................................................................................32 Slag Attack Test ...........................................................................................................................33 Hot Metal Resistance test ............................................................................................................33 Hot Metal Penetration test ..........................................................................................................34. Characterization of porous materials...................................................................................................37 4.1. 4.2. 4.3. 4.4.. 5.. Alkali attack .................................................................................................................................25 Zinc attack ...................................................................................................................................26 Water vapor oxidation .................................................................................................................26 Deterioration by carbon monoxide ..............................................................................................27 Slag attack ...................................................................................................................................27 Erosion and penetration by the molten metal ..............................................................................27 Degradation of the blast furnace hearth ......................................................................................29. Laboratory tests for carbon and graphite refractory materials degradation .........................................31 3.1. 3.2. 3.3. 3.4.. 4.. Construction and operation principles .........................................................................................17 Refractory materials used in the blast furnace .............................................................................19 Blast furnace hearth lining designs ..............................................................................................21 Carbon and graphite refractories .................................................................................................21. Porosity and pores size distribution .............................................................................................37 Permeability ................................................................................................................................38 Tortuosity ....................................................................................................................................40 Constrictivity ...............................................................................................................................40. Methods for investigating porous materials ........................................................................................43 5.1.. Volume averaged parameters......................................................................................................43. 5.1.1. 5.1.2. 5.1.3. 5.2.. Morphology analysis....................................................................................................................48. 5.2.1. 5.2.2. 5.2.3. 6.. Morphology in 2D ................................................................................................................49 Morphology in 3D ................................................................................................................49 X-ray computed tomography ...............................................................................................50. Theoretical description of transport in porous materials .....................................................................55 6.1. 6.2.. Pore network models ..................................................................................................................55 Continuum models ......................................................................................................................56. 6.2.1. 6.2.2. 6.3.. Micro-scale approach — pore-scale method ........................................................................57 Macro-scale approach..........................................................................................................58. Calculations of porous materials properties .................................................................................59. 6.3.1. 6.3.2. 7.. Mercury intrusion porosimetry ............................................................................................43 Helium porosimetry .............................................................................................................45 Gas permeability ..................................................................................................................46. Representative elementary volume .....................................................................................59 Calculation of permeability ..................................................................................................60. Aim of the thesis .................................................................................................................................65.

(6) 6. 3D pore structure and infiltration resistance of micropore carbon materials. 8.. Methodology of experimental investigations ......................................................................................67 8.1. 8.2.. Glossary ......................................................................................................................................67 Materials .....................................................................................................................................67. 8.2.1. 8.2.2. 8.3.. Volume averaged parameters methods .......................................................................................69. 8.3.1. 8.3.2. 8.3.3. 8.4. 8.5.. Mercury intrusion porosimetry ............................................................................................69 Helium porosimetry .............................................................................................................70 Gas permeability ..................................................................................................................71. 3D pore structure investigation using X-ray computed tomography .............................................71 Algorithms of pores reconstruction — processing of XCT data .....................................................72. 8.5.1. 8.5.2. 8.5.3. 8.5.4. 8.5.5. 8.5.6. 8.5.7. 9.. Samples for heat treatment investigations ...........................................................................68 Samples for molten metal infiltration investigations ............................................................69. Pre-processing .....................................................................................................................72 Separation of the pore structure ..........................................................................................72 Separation of the continuous pores .....................................................................................73 Porosity calculation..............................................................................................................73 Local pore thickness calculations .........................................................................................73 Tortuosity calculations .........................................................................................................74 Constrictivity calculations ....................................................................................................75. Model of the molten metal pore-scale flow in carbon refractory materials..........................................77 9.1. 9.2. 9.3. 9.4. 9.5.. 10.. Stationary model of the liquid metal pore-scale flow ...................................................................77 Generation of 3D pore geometry and mesh .................................................................................78 Numerical method.......................................................................................................................79 Simulation results ........................................................................................................................80 Calculation of permeability ..........................................................................................................80 Helium porosimetry ........................................................................................................................85. 10.1. Material A ...................................................................................................................................85 10.2. Material B ...................................................................................................................................85 10.3. HP summary ................................................................................................................................86 11.. Gas permeability .............................................................................................................................89. 11.1. Material A ...................................................................................................................................89 11.2. Material B ...................................................................................................................................90 11.3. GP summary ................................................................................................................................90 12.. Mercury intrusion porosimetry........................................................................................................93. 12.1. Material A ...................................................................................................................................93 12.1.1. 12.1.2. 12.1.3.. Mercury porosity and pore size distribution .........................................................................93 MIP tortuosity......................................................................................................................94 Permeability based on MIP measurements ..........................................................................95. 12.2. Material B ...................................................................................................................................97 12.2.1. 12.2.2. 12.2.3.. Mercury porosity and pore size distribution .........................................................................97 MIP tortuosity......................................................................................................................98 Permeability based on MIP measurements ..........................................................................99. 12.3. MIP summary ............................................................................................................................101 12.3.1. 12.3.2. 12.3.3.. Mercury porosity ...............................................................................................................101 MIP tortuosity....................................................................................................................102 Permeability ......................................................................................................................102.

(7) Content 13.. 7. X-ray computed tomography .........................................................................................................105. 13.1. Material A .................................................................................................................................105 13.1.1. 13.1.2. 13.1.3. 13.1.4.. Total and continuous porosity ............................................................................................105 Local pore thickness ...........................................................................................................115 XCT tortuosity ....................................................................................................................122 Constrictivity......................................................................................................................122. 13.2. Material B .................................................................................................................................124 13.2.1. 13.2.2. 13.2.3. 13.2.4.. Total and continuous porosity ............................................................................................124 Local pore thickness ...........................................................................................................134 XCT tortuosity ....................................................................................................................141 Constrictivity......................................................................................................................141. 13.3. XCT summary ............................................................................................................................143 13.3.1. 13.3.2. 13.3.3. 13.3.4. 14.. Total and continuous porosity ............................................................................................143 Local pore thickness ...........................................................................................................144 XCT tortuosity ....................................................................................................................148 Constrictivity......................................................................................................................149. Pore-scale flow simulations ...........................................................................................................151. 14.1. Results of the simulations ..........................................................................................................151 14.2. Permeability calculations ...........................................................................................................157 14.3. Summary of pore scale flow simulations ....................................................................................158 15.. Investigations of molten metal infiltration .....................................................................................159. 15.1. 15.2. 15.3. 15.4. 16. 17.. Modification of the HMP test — HMP-XCT.................................................................................159 Material A .................................................................................................................................160 Material B .................................................................................................................................164 HMP-XCT summary....................................................................................................................168. Permeability — experimental measurements and numerical simulations ......................................173 Various parameters describing porous materials ...........................................................................179. 17.1. 17.2. 17.3. 17.4.. Open porosity............................................................................................................................179 Permeability ..............................................................................................................................182 Tortuosity ..................................................................................................................................185 Constrictivity .............................................................................................................................187. 18. Summary and conclusions .............................................................................................................189 19. Future ...........................................................................................................................................195 List of figures ............................................................................................................................................207 List of tables .............................................................................................................................................211 Appendix 1 ...............................................................................................................................................213 Appendix 2 ...............................................................................................................................................219.

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(9) Preface Steel is one of the most widespread materials in the world, present in almost all branches of industry, from infrastructure, construction and transport to packing and machinery. The steel demand in 2019 was equal to 1.65 billion of tonnes [1]. Despite the growing popularity of the recycling methods, the blast furnace (BF) technology still remains the main method for obtaining crude iron, which is a substrate for steel production. There is a constant ratio of 0.7 tonne of crude iron for 1 tonne of the steel. Thus, extending the lifetime of blast furnaces has a significant influence on the cost of steel [2]. One of the factors affecting the lifetime of the blast furnace is a degradation of BF lining materials. During the BF campaign, the refractory materials are subjected to different degradation mechanisms, dependent on the zone of BF, in a particular material is located. Carbon and graphite materials, due to their unique properties, like thermal stability, high thermal conductivity, corrosion resistance and non-wettability, are widely used as lining materials in the BF [3]. Micropore carbon materials are used in the BF hearth, where the crude iron gathers. Due to its presence, one of the most significant degradation factors is wear, related to the infiltration of liquid metal into the refractory material and its dissolution. Better understanding of the degradation due to liquid metal infiltration might be used for design of a more resilient materials, allowing the cost of the BF process and, consequently, the price of steel to be reduced. The following thesis focuses on the description of the pore structure and transport properties of micropore carbon refractory materials used in the blast furnace hearth. These properties strongly affect materials degradation resistance in contact with molten metal and thus have influence on their durability and, as a result, on the lifetime of blast furnace. The thesis is divided into four parts: . Introduction. Background and literature review (Chapters 16). . Objective and motivation. Aim and methodology (Chapters 79).. . Results (Chapters 1015).. . Discussion. Summary and conclusion (Chapters 1619).. Chapter 1 (Blast Furnace) shows the basics of the blast furnace technology, i.e. operation principles and furnace construction. Different designs of a blast furnace hearth are presented. The blast furnace lining materials are described, in particular various types of carbon and graphite refractory materials used in the zone of the blast furnace hearth, where liquid metal gathers. In Chapter 2 (Mechanisms of blast furnace hearth degradation) the major mechanisms of blast furnace hearth degradation are discussed separately. These processes include: alkali attack, water vapor oxidation, deterioration by carbon monoxide and degradation in contact with molten crude iron and slag. The chapter is concluded by the description of the overall degradation process, which is a result of all degradation processes occurring simultaneously. Chapter 3 (Laboratory tests for carbon and graphite refractory degradation) presents the idea and experimental setups of the most common laboratory test design for investigations of the processes described in Chapter 2. The tests used, in particular, for investigating of carbon and graphite refractory materials are described, e.g. Alkali Attack test, Slag Attack test, Hot Metal Resistance and Hot Metal Penetration tests (HMP). Resistance to the degradation resulting from molten metal infiltration is strongly dependent on the pore structure of refractory materials. Chapter 4 (Characterization of porous materials) focuses on the properties that describe the pore structure and transport properties of porous materials such as: porosity, pore size distribution, permeability, tortuosity and constrictivity. Chapter 5 (Methods of investigating porous materials) contains the description of experimental investigation methods, which can be used to measure the properties presented in Chapter 4. The methods are divided into two groups. The first group focuses on volume average parameters methods, e.g. mercury.

(10) 10. 3D pore structure and infiltration resistance of micropore carbon materials. intrusion porosimetry, helium porosimetry and gas permeability. The other one presents the methods for morphology analysis in 3D, and in this section the X-ray computed tomography method is described in details. Chapter 6 (Theoretical description of transport in porous materials) introduces the different approaches of describing porous materials: pore network and continuum models. The continuum models are described in two scales: pore-scale and macro-scale. The idea of a Representative Elementary Volume (REV) is presented together with the methodology for calculating effective transport properties, i.e. permeability based on the solution of pore-scale flow models. Chapter 7 (Aim of the thesis) presents the aim of this work. The main goal of the thesis is formulated as well as a set of lesser objectives, which were used as a starting point for designing the experimental part. Chapter 8 (Methodology of experimental investigations) presents a description of the experimental investigations carried out in this thesis. The chapter includes: a glossary of various parameters describing the same property (e.g. helium, mercury and continuous porosity); description of investigated micropore carbon materials and the information about the samples coded into their names; details of the experimental methods used in this research, i.e. gas permeability, helium porosimetry, mercury intrusion porosimetry and X-ray computed tomography. The methodology of XCT data processing is presented in details, including: preprocessing, separation of total and continuous porosity, calculation of pore size distribution, local thickness, tortuosity and constrictivity. Chapter 9 (Model of molten metal pore-scale flow in carbon refractory materials) describes the assumptions upon which the pore-scale flow model was formulated. The equations describing molten metal motion and boundary conditions are presented. The methods used to create 3D geometries and 3D meshes for numerical simulation are shown as well as the method used to calculate the permeability based on the results of pore-scale flow simulations. Chapters 10-13 (Helium porosimetry, Gas permeability, Mercury Intrusion Porosimetry and X-ray computed tomography) present the various parameters describing micropore carbon materials measured using those methods. These chapters focus on the influence of the heat treatment on the investigated properties, the differences between foot and head zones of micropore carbon blocks as well as the differences between the experimental (A) and standard (B) refractory materials analyzed in the thesis. Chapter 14 (Pore-scale flow simulations) shows the results of the numerical simulations performed for the selected 3D continuous pore structures of material A, measured using XCT. The influence of the heat treatment on the pore structure, and further on the flow of molten metal is described based on the changes in the flow paths, maximum and average velocities and permeabilities of the analyzed pore structures. Chapter 15 (Investigations of molten metal infiltration) presents an elaborated modification of the standard HMP test by expanding the test with XCT measurements. The created test (HMP-XCT), which allows observing the changes in the 3D pore structure resulting from the molten metal infiltration as well as identifying the preferred paths for metal infiltration, is described in the chapter. The example results for both materials A and B are also presented. Chapter 16 (Permeability —experimental measurements and numerical simulations) focuses on the comparison of the measured gas permeabilities (KGP) and calculated pore-scale flow permeabilities (KPSF), which were measured for the exactly same samples. The differences between the measured and calculated values are summarized and discussed. The formula that links those two permeabilities is presented. Chapter 17 (Various parameters describing the porous materials) summarizes and compares the results presented in the previous chapters, mainly Chapters 1013. This chapter focuses on the four parameters investigated in the thesis: open porosity, permeability, tortuosity and constrictivity. Each parameter was measured using different methods that lead to the different values of the same property. The differences between the obtained results are compared and explained based on the principles of the used experimental methods..

(11) Preface. 11. Chapter 18 (Summary and conclusions) presents the main conclusions of the thesis. This chapter focuses on the major achievements of the thesis and on the comparison of the experimental (A) and standard (B) micropore carbon refractory materials. The usefulness and application of the experimental methods and numerical simulations are summarized. Chapter 19 (Future) presents possible extensions of the work presented in the thesis, which are divided into two categories. The first, numerical category, focuses on the extension of the pore-scale flow simulations in 3D geometry, representing the pore structure of the micropore carbon material. The model of an evolutionary two-phase flow is presented, which can be used to simulate the molten metal infiltration more accurately taking into account the interaction between the molten metal and the carbon material. The second, experimental category, presents the idea of utilizing the non-destructive measurements (XCT) in the investigations of other degradation mechanisms, e.g. zinc and alkali attack..

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(13) List of abbreviations General: BF — blast furnace HMP — Hot Metal Penetration test HMP-XCT — modified Hot Metal Penetration test HMR — Hot Metal Resistance test SAT — Slag Attack test AAT — Alkali Attack test Samples: Ref — reference sample HT — sample after the heat treatment HMP — sample after the hot metal penetration test Methods of investigations: XCT — X-ray computed tomography GP — gas permeability HP — helium porosimetry MIP — mercury intrusion porosimetry PSD — pore size distribution LPT — local pore thickness LPTSD — local pore thickness size distribution CFD — computational fluid dynamics PSF — pore-scale flow simulations.

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(15) Introduction Background and literature review.

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(17) 1. Blast Furnace The blast furnace (BF) is a part of the steelmaking technology, in which pig iron, also known as crude iron, is obtained from iron ore. It is widely considered that the BF is a central and essential part of the of the steelmaking process — the typical steelworks consist of: coke plant, sintering plant, blast furnaces, steel plant, rolling mill, power plant and oxygen plant. While iron ore, pellets and coke can be provided by subcontractors, the blast furnace is necessary to reduce the iron compounds. The origins of the BF technology can be found in China in 200 BC, while in Europe it was first introduced in the Middle Ages. After the Industrial Revolution, the demand for steel increased greatly, and thus the improvements in the production efficiency of the BF were required. The significant development of the BF technology started in the 18th and 19th centuries with the utilization of coke and steam engines instead of charcoal and water wheels and by introducing the hot air blast. The further research and development increased the size, efficiency, and productivity of blast furnaces — from 4 tons per day in 1795 to over 14,000 tons per day for the modern BF in 2013 [4,5]. The chapter will present the operation principles and the construction of the BF. The designs of BF lining materials will be described, with major focus on carbon and graphite materials.. 1.1. Construction and operation principles The blast furnace is a continuously operating, large metallurgical shaft furnace designed to carry out the physical conversion and chemical reduction of iron oxides into molten pig iron. The volume of the modern BF is above 4,100 m3, which leads to the production yield of more than 14,000 tons of hot metal per day, while the expected lifetime of the BF is twenty or even more years [6,7]. The scheme of the BF construction is presented in Fig. 1.1a. Seven major parts can be distinguished in its construction:  throat — a straight cylinder in the upper part of the furnace — provides a symmetrical distribution of the load inside the furnace;  stack (known also as a shaft) — a truncated cone, expanding downwards — transport the burden, the volume of which increases due to heating, downwards;  belly — a straight cylinder — provides a smooth transition between the widening stack and tapered bosh;  bosh — a truncated cone, tapering downwards — makes transport of liquid products to the hearth easier;  tuyere zone — 15 to 50 nozzles (depending on the diameter of the hearth) in the upper end of hearth, through which the hot air is blown into the furnace;  hearth — a straight cylinder at the bottom of the furnace, which gathers the liquid products;  taphole — a set of pipes located at 1/3 of hearth height — to tap the gathered pig iron. The number of tapholes depends on the size of the furnace. The blast furnace wall is made of three main successive layers: i) external steel shell, ii) cooling system and iii) the internal refractory brickwork. The cooling system is made of two separate streams at the top and at the bottom part of the BF. The former is made of cast iron staves, while the latter is made of copper staves. Additionally, the spray cooling of steel shell is also used at the bosh and the hearth. Cooling is necessary to maximize the BF working capacity and increase the lifetime of the BF lining materials. [5,7,8]. The blast furnace operates on the counter-current principle — the solid raw materials, such as iron oxides (in form of pellets or lump ore), coke and fluxes (e.g. limestone and dolomites), are supplied in layers from the top of the BF, while hot air required for the reduction is blown via the tuyeres in the bottom of the furnace. There are two major flows in that process. First is a hot gas, generated by the burning of the coke, which flows up the furnace, melting and reducing the iron oxides. The second is a much slower flow of solids descending towards the bottom of the furnace [9]. The gaseous products (mainly CO2, CO, N2, and dust) are.

(18) 18. 3D pore structure and infiltration resistance of micropore carbon materials. removed from the BF through the throat and then used to preheat the air blast or to generate electricity. The liquid products, i.e. the pig iron and the slag, are removed from the BF via the tapholes [7]. As a result of those processes, the temperature inside BF increases towards the bottom.. Fig. 1.1 a) Scheme of the blast furnace construction, b) BF profile with marked temperatures ranges for: 1 — evaporation of moisture, 2 — carbonates decomposition, 3 — iron oxides reduction, 4 — manganese oxide reduction, 5 — phosphorus compounds reduction, 6 — slag melting, 7 — desulphurisation and reduction in liquid phase, 8 — coke combustion, 9 — silicon compound reduction, 10 — carburization [5].. The non-uniform temperature distribution results in the fact that various processes, e.g. reduction of specific compounds occur in particular zones. The scheme of the temperature profile and processes that occur inside the BF is presented in Fig. 1.1b. Despite the complexity of all processes that take place inside the BF, the whole process can be summarize by the six major reactions presented in Table 1.1. Table 1.1 Main reaction inside the blast furnace [10].. Reaction. ΔH [kJ/mol]. C + O2  CO2. (1). - 406.12. C + CO2  2CO. (2). + 172.47. FeO + C  Fe + CO. (3). FeO + CO  Fe + CO2. (4). + 158.34 - 17.13. CO + 3Fe2O3  2Fe3O4  CO2. (5). - 52.85. CO + Fe3O4  3FeO + CO2. (6). + 34.46. The descending burden is dried and preheated by the ascending gas. The hot air, enriched in oxygen, is blown through the tuyeres, and the combustion of the coke (1) takes place in the zone closed to the tuyeres, called raceway. It is the most exothermic reaction, which results in the highest temperatures in the BF — between 2100 and 2300 oC. The produced CO2 reacts with the coke and produces CO in the process called the Boudouard reaction (2). The ascending gas flows through the cohesive zone, known also as the softeningmelting zone, in which iron and slag become soft, separate and melt down. The molten slag contains a certain amount of unreacted FeO, which is reduced by solid carbon in the process called direct reduction (3). Below.

(19) Blast Furnace. 19. the cohesive zone is the active coke zone, where the final reduction to metallic compounds is completed, and there is only solid coke, liquid iron and slag. The produced carbon monoxide is the main reduction agent in the BF — it reacts with FeO, in the process called indirect reduction (4), which occurs at temperatures above 900 oC. However, the iron oxide in the burden is not in the form of wustite (FeO) but mainly in the form of hematite (Fe2O3) and magnetite (Fe3O4). Below the temperature of 570 oC, the conversion of hematite into magnetite occurs (5), while above 570 oC, magnetite is reduced into wustite (6). The inactive unburned coke particles form a porous packed bed, called the deadman, which flows on the slag. The deadman has influence on the metal flow inside the BF hearth and, as a consequence changes the distribution of the temperature in the BF walls. The molten pig iron and slag, which gather in the BF hearth, are removed from the BF by the tap hole, 6-10 times per day, or continuously if the BF has more than two tapholes. The main product of the BF is pig iron with a composition of up to 4.5 wt% of C, 0.3  0.7 wt% of Si, 0.2 - 0.4 wt% of Mn and 0.06 - 0.13 wt% of P. The composition of the obtained pig iron is then refined, using a basic oxygen furnace, to reduce the excessive amounts of C, Si, Mn, Ti, V and P. The second (side) product of the BF process is slag, which is a mixture of the molten oxides. Its four main components are: CaO, SiO2, Al2O3 and MgO. The remaining components are MnO, TiO2, K2O, Na2O and P. Due to its lower density, molten slag gathers on the top of the pig iron, and it is tapped at the same time. Despite the fact that BF slag is a side product, it has found a broad application as a construction material in road building and as a concrete aggregate in bituminous surfaces. However, its major application is a result of its hydraulic properties — it is part of 14 different types of cements, according to the current European cement regulations [5,7,11–14].. 1.2. Refractory materials used in the blast furnace The blast furnace is a complex reactor, in which many different processes occur, depending on the zone of the furnace. In general, the refractory materials can be divided into two groups: oxide ceramics, called the white or isolation solution, and carbon and graphite materials, called the black or thermal solution. The former group is based on the idea of using materials with low to medium thermal conductivity to minimize the heat loss. These materials possess a very high resistance to simultaneous mechanical, thermal and chemical stresses, resulting from the BF inner environment. This group includes materials such as: mullite, corundum, high alumina, alumina-chrome and alumina-carbon bricks. Using the white solution, the price of the refractory materials is higher, but the price of the cooling system can be lower. A lower thermal conductivity results in lower energy losses through the BF walls, and thus the fuel consumption in the process is also lower. The latter group of the materials is based on the idea of using materials with a medium to high thermal conductivity. In this approach, the reaction limit temperature, which is the temperature at which the material becomes chemically active in the furnace, is lowered and thus the chemical wear is reduced. This group includes materials such as: amorphous carbon, supermicropore and micropore carbon, semigraphite, and graphite. Using the black solution, the price of the refractory materials is lower, but the price of the cooling system is higher. The higher thermal conductivity results in higher energy losses through the BF walls, and thus the fuel consumption in the process is also higher. Besides the two presented groups, the third group can distinguished, which has intermediate properties. It consists of materials such as silicon carbide, silicon nitride, sialons (SiAlON) and their combinations. Despite the two major approaches in the design of the BF linings, in modern furnaces the combinations of materials with different thermal conductivities are used. In general, oxide materials are used in the upper parts of the furnace, while carbon and graphite materials are mainly used in the BF hearth [3,8,15]. The comparison of various refractory materials used in the different zones of the BF is presented in Table 1.2..

(20) Table 1.2 Refractory materials used at various zones of the blast furnace [8].. Blast furnace zone Throat. Stack. Belly. Bosh. Tuyeres. Hearth Wall. Taphole. Al2O3-SiO2*. Graphite. Carbon. Carbon. Carbon. Carbon. Carbon. SiC. Semigraphite. Graphite. Graphite. Graphite. Graphite. Graphite. Hot pressed carbon Hot pressed semigraphite. Hot pressed carbon Hot pressed semigraphite. Hot pressed carbon Hot pressed semigraphite. Al2O3-SiO2*. Hot pressed carbon. Hot pressed carbon. Al2O3-Cr2O4. Al2O3-SiO2*. Al2O3-SiO2*. Al2O3-Cr2O4. Al2O3-SiO2*. Al2O3-SiO2*. Al2O3-SiAlON. Al2O3-SiAlON. Al2O3-Cr2O4. Al2O3-Cr2O4. SiC-SiAlON. Al2O3-SiAlON. SiC-C SiC. Al2O3-SiO2*. *. Semigraphite Semigraphite Semigraphite Semigraphite Semigraphite. Ceramic pad Al2O3SiO2* Al2O3Cr2O4 Al2O3SiAlON SiCSiAlON. Upper bottom of hearth. Lower bottom of hearth. Carbon. Carbon. Graphite. Graphite. Semigraphite Semigraphite Hot pressed carbon Hot pressed semigraphite. Hot pressed carbon Hot pressed semigraphite. Al2O3-Cr2O4. Al2O3-SiO2*. Al2O3-SiO2*. SiC-SiAlON. Al2O3-SiAlON. Al2O3-SiAlON. SiC-C. Al2O3-SiAlON. SiC-SiAlON. SiC. SiC-C. SiC-SiAlON. SiC-SiAlON. SiC-C. SiC-C. SiC-C. SiC. SiC. SiC. Note that Al2O3-SiO2 is a broad group of the materials with a different compound ratios. The amount of SiO2 is in the range between 30 and 90 %..

(21) 21. Blast Furnace. 1.3. Blast furnace hearth lining designs One of the most essential part of the blast furnace lining, which has a significant influence on its lifetime, is the BF hearth. While the other parts of the BF lining can be replaced in a relatively short period of time, the relining of the hearth is much more complicated and results in a complete revamp of the furnace. Thus, it is a commonly accepted that the lifetime of the BF is determined by the erosion and corrosion of the BF hearth [16–18]. The design of the BF hearth lining has been continuously improved. The first concepts of high alumina bricks and a chamotte hearth bottom with no cooling systems have evolved into baked carbon blocks combined with a high alumina hearth bottom with a cooling system. The thermal conductivity, oxidation resistance and strength of used materials have been improved gradually. With the later design of the French ceramic cup and American hot-pressed small carbon bricks, the two concepts, i.e. all-carbon brick and ceramic cup hearth bottom, have become the most widespread designs. There were considered as the method of heat transfer and heat isolation, respectively. While there were some disagreements between them, the former resulted in high heat losses, and the latter was expensive, but the utilization a high thermal conductivity ensured a longer lifetime of the BF hearth [19]. Nowadays, four different hearth designs are used. There are presented in Fig. 1.2. The main materials in all of them are carbon-based blocks and bricks combined with high quality ceramics. Due to a long expected lifetime, the price of the lining is less important than its quality and durability.. Fig. 1.2 Typical BF hearth constructions: a) and b) European, c) Japanese and d) North American [20].. Fig. 1.2a presents one of the European hearth designs in which walls are made of micropore carbon blocks and, in the upper part, of amorphous carbon bricks. Between them and the steel shell, highly conductive graphite bricks are used. The bottom is made of horizontally installed amorphous carbon blocks and graphite blocks underneath. In the second European design (Fig. 1.2b), the straight wall is made of micropore carbon blocks and the external layer of graphite. The hearth bottom has three chamotte layers in the upper part and micropore and amorphous carbon underneath. It is also protected by a ceramic cup. The shell has a large inclination angle in order to create thicker lining in the critical regions. The Japanese hearth design (Fig. 1.2c) makes use of micropore carbon blocks with a cupshaped bottom. It has a ceramic cup and amorphous carbon blocks below it, installed vertically to minimize joints. In all three designs, the inner part of the lining is made of a thin layer of high alumina bricks. The last, North American design (Fig. 1.2d) has thin vertical walls made of small carbon bricks. The hearth bottom is made of three layers: small chamotte bricks, horizontal amorphous carbon blocks and a graphite pad. Unlike in the three previous designs, there is no bottom cooling [17,20].. 1.4. Carbon and graphite refractories Carbon and graphite refractory materials are commonly used as lining materials in lower parts of the blast furnace, i.e. in the BF hearth. Their broad application is a consequence of their unique properties. They:  are the solids that exhibit the highest temperature stability,.

(22) 22. 3D pore structure and infiltration resistance of micropore carbon materials   . do not melt and deform at high temperatures. They evaporates without melting above 3600 oC; are not-wetted by most of metals and slag; have better mechanical properties at elevated temperatures. The strength of graphite increases with temperatures up to 2500 oC;  have low thermal expansion coefficients, and thus a great thermal shock resistance;  have a high electrical and thermal conductivity;  they are inert to most chemical compounds, except strong oxidants;  can be oxidized; the process starts at 400 oC in the presence of free oxygen [3,21]. Carbon and graphite refractory materials form a group of the materials, among which several types of materials can be distinguish, such as amorphous carbon, micropore and super micropore carbon, semigraphite, semigraphitized, graphite and hot-pressed carbon graphite. Despite their different raw materials composition, and thus final properties, there are a similarities in their technologies. All of them are obtained from a mixture of carbonaceous filler (e.g. coke or graphite) with a carbon binder (e.g. coal tar pitch or resin) with possible additives. Then, the form products are baked, which results in the carbonization of the binder. After baking, these materials can be further modified, for example, by impregnation or annealing at higher temperatures [22,23]. A description the most common carbon and graphite materials is presented below. Amorphous carbons (also known as formed or baked carbon) are materials in which the carbonaceous filler, i.e. calcined anthracite coal, petroleum coke or carbon black, is mixed with petroleum pitch, coal tar pitch or phenolic resin as a binder. The mixture is formed by vibromolding or extrusion and then conventionally baked in ring furnaces or car bottom furnaces at temperatures from 800 to 1400 oC. Baked materials contain carbon particles with a carbon binder. In order to decrease the open porosity resulting from the carbonization of binder, materials can be further impregnated with the binder and rebaked, yet it also increases the cost of these materials. [24,25] Micropore and supermicropore carbon materials are a modification of amorphous carbon materials. The initial recipe, apart from carbonaceous fillers and carbon binder, includes alumina grains and silicon metal powder. Products are also baked in furnaces at temperatures from 800 to 1400 oC. The addition of alumina grains increases the corrosion resistance in contact with molten iron, while silicon powder decreases the porosity larger than the 1 µm. During the baking process, the initial silicon powder reacts with the carbon, and creates the silicon carbide whiskers which clog the pores. Examples of the created SiC whiskers structure are presented in Fig. 1.3. The distinction between supermicropore and micropore materials was proposed by the manufactures, based on the fraction of pores larger than 5 µm measured using mercury intrusion porosimetry. Micropore materials have the cumulative volume of pores larger than 5 µm between 5% to 2%, while supermicropore materials have it below 2% [16,24,26,27].. Fig. 1.3 SiC whiskers in supermicropore carbon materials at magnification: a) 5 000 x and b) 100 000 x [27]..

(23) Blast Furnace. 23. Semigraphites are materials in which artificial graphite is used as a carbonaceous filler, and it is mixed with carbon binders, such as pitch or resin. These materials are formed using vibromolding or extrusion and are baked at temperatures between 800 and 1400 oC. The resulting product is made of graphite grains bounded with an amorphous carbon matrix, and thus it has a higher thermal conductivity than amorphous carbon materials and lower than pure graphite materials. Like in the case of baked carbon materials, its properties might be improved by the impregnation with a binder or by the addition of alumina grains and silicon powder. Resulting products can be considered as micropore or supermicropore semigraphite materials [24]. Graphites are materials that are produced in two steps of the heat treatment. First, the formed blocks made of a mixture of cokes and a carbon binder are baked at a temperature between 800 and 1200 oC. Then, the obtained block, made of a carbon filler and binder, is subject to the second step of heat treatment, called graphitization. It is performed at temperatures between 2400 and 3000 oC, using Acheson or Castner furnaces. During this process, the initial amorphous turbostratic structure is transformed into a crystalline graphite structure. The process changes the structure of both filler grains and the binder. In order to decrease the porosity resulting from the carbonization of the binder, graphite materials can be impregnated with pitch and regraphitized [23,24]. Semigraphitized are amorphous carbon materials that after baking are subject to heat treatment at temperatures between 1600 and 2400 oC. The second heat treatment alters the turbostratic structure of carbon materials. However, due to the fact that it is performed at a temperature below the graphitization temperature, the semigraphitized materials are not crystalline. Compared to amorphous carbon and semigraphite materials, they have a higher thermal conductivity and resistance to chemical attack [24]. The last group of the materials are hot-pressed carbon and graphite materials. Compared to the previously described materials, hot-pressed materials have a different forming and baking method, i.e. those processes are not separated. The mixture of carbonaceous particles and binder are introduced into special mold. The hydraulic ram, pressurizes the mixture, while an electric current passes through the mold and carbonizes the binder. The baking process takes minutes and provides very dense materials. However, it is very expensive and only small bricks can be produced [20,24]. A comparison of selected properties of various carbon and graphite materials is presented in Table 1.3. Table 1.3 Raw materials and the properties of selected carbon and graphite materials [28].. Type of material amorphous carbon micropore carbon super micropore carbon semi-graphite. micropore semi-graphite graphite. Raw basis materials. Microporosity. Pig iron and slag resistance. Thermal conductivity. Cold crushing strength. anthracite, coke. low. low. low. medium. anthracite, silicon carbide. high. medium. low. high. anthracite, silicon carbide, alumina. very high. very high. low. very high. high. low. medium. medium. very high. high. high. high. high. low. very high. low. electrographite, calcined coke powder electrographite, calcined coke powder, silicon carbide, alumina graphite.

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(25) 2. Mechanisms of blast furnace hearth degradation One of the major factors that affect the lifetime of the blast furnace is a degradation of refractory materials used as the lining. The degradation of lining materials depends on their quality, the composition of the burden, the cooling system and the operation of the BF. The proper design of the BF lining requires knowledge of the various degradation mechanisms which occur inside the BF. Due to the complexity of the processes in the BF, different degradation mechanisms are present in the each zone of the BF. In general, they can be divided into three groups:  mechanical wear, e.g. abrasive wear of descending solid burden materials, impact load from falling burden, erosion from ascending dust laden gases in the top part of the BF and the erosion of the lining in a BF hearth resulting from the flow of molten metal;  thermal wear, e.g. thermal shocks caused by the many tapping cycles and thermo-mechanical stresses resulting from the temperature gradients;  chemical wear, which is a results of the interaction at high temperatures between various compounds present in the BF and the lining materials, e.g. attack by alkali vapor, carbon monoxide degradation, oxidation and attack by molten metal and slag [24,29]. As it was presented in the previous chapter, the lifetime of the BF is mainly dependent on the degradation of the lining in a BF hearth. The current chapter will focus on the various degradation mechanisms that affect the lining materials in that area. In the end, the mechanism of their combined influence on the degradation of lining materials, will be presented.. 2.1. Alkali attack The source of alkalis (mainly potassium and sodium hydroxides) in the BF is a burden. They are contaminants in iron ore, coke ashes and in some fluxes. Their main source is metallurgical coke — about 50% of incoming alkalis are from it. The alkalis are introduced into the BF in the form of carbonates, cyanides and silicates. During the BF process, only approx. 8% remain inside the furnace, while the rest are removed via slag (90%) and dust. The allowed limit of alkalis in the burden, which depends on the blast furnace and operation principles, is between 2.5 to 8.5 grams per 1 kg of pig iron. Postmortem analysis of the BF lining materials showed that 80% of total alkali amount is potassium [30,31]. The alkali attack starts with the penetration of alkaline metal vapors into the open porosity of refractory materials. The penetration is limited to the 800 oC isotherm, which is the condensation temperature of alkaline vapors. The alkalis can interact with the: carbon and graphite grains and a binder, or with the mineral additives, such as alumina, silica and silicon carbide. In the interaction with the carbon and graphite, the alkalis intercalate between carbon lamellae and create carbon potassium compounds, such as C8K, C24K and C60K. As a result, inter-granular stresses are generated, which cumulate and create the micro cracks. This type of alkali attack depends on the type of carbon aggregates in the refractory materials — aggregates with a more graphitic structure are less susceptible to intercalation than the more amorphous ones [29,32]. In the interaction with the mineral additives, the metallic vapors are oxidized by the carbon monoxide and form oxides: (7) 2K + CO2  K2O + CO The potassium oxide deposited on the lining further reacts with the alumina and silica within the material forming kaliophilite (8), which further reacts with the silica and produces leucite (9): (8) K2O + Al2O3 + 2SiO2(s)  K2O  Al2O3  2SiO 2. K2O  Al2O3  2SiO 2 + 2SiO2  K2O  Al2O3  4SiO 2. (9).

(26) 26. 3D pore structure and infiltration resistance of micropore carbon materials. The metallic vapors might also react with silicon carbide and alumina in the presence of carbon monoxide and produce liquid leucite: (10) 2K + 4SiC + Al2O3  9CO  K2O  Al2O3  4SiO 2  13C Potassium is the main component of alkalis in the BF, thus the formation of kaliophilite and leucite is a dominant process. However, beside potassium, sodium is also present, which results in the formation of other compounds, e.g. nepheline — (Na,K)2O ∙ Al2O3 ∙ 2SiO2 [33]. The formation of new phases with a larger volume and deposition of carbon induces mechanical stresses, which leads to the formation of micro cracks. As a result, the microstructure of the refractory material becomes disintegrated, which lowers its thermal conductivity and shifts the condensation isotherm towards outer parts of the refractory brick, which accelerates the degradation process [18,34]. Materials resistance to the alkali attack can be improved by changes in materials pore structure. According to Xu et al. [34], materials with a lower permeability have better alkali corrosion resistance than materials with a higher permeability, as the reduction of pore sizes limits the penetrations of alkalis vapors into materials.. 2.2. Zinc attack Zinc is introduced into the BF with the burden, as part of the iron ore, pellet and sinter as well as coke. Its amount is higher in the sinter and pellets than in the natural ore, which is a result of the frequent processing of BF waste materials. Zinc is delivered in the form of oxide, ferrite, silicate or sulfide [35,36]. The mechanism of zinc attack is similar to the alkali attack. The zinc compounds are reduced, and the metallic zinc evaporates and penetrates the open pores in the refractory materials. Close to 800 oC, zinc condensates and is reduced by the carbon dioxide: (11) Zn + CO2  ZnO + CO The formation of zinc oxides is associated with the volume expansion by approx. 57%, which creates mechanical stresses. This results in the formation of micro cracks. Moreover, the deposited zinc oxide might react with alumina and silica additives present in the carbon materials and produce zinc aluminate (12) and zinc silicate (13):. ZnO + Al2O3  ZnAl2O4. (12). ZnO + SiO2  ZnSiO3. (13). which results in further volume expansions [18,37–39]. There are some differences in the literature as regards zinc silicates composition; some authors report the formation of ZnSiO3 [18,37], while others of Zn2SiO4 [35,38,39]. The formation of zinc oxide, aluminate and silicate, similar to the deposition of alkalis oxides and alumino-silicates, results in the disintegration of the refractory materials, which lowers the thermal conductivity and accelerates the degradations of the lining materials.. 2.3. Water vapor oxidation One of the characteristic features of carbon and graphite materials is their very low oxidation resistance. Therefore, these materials are used as lining in areas where there is no oxidizing atmosphere, e.g. in a BF hearth. In the zones, where free oxygen is present, e.g. in the tuyeres zone, oxide materials are used. However, carbon and graphite materials might be subject to water vapor oxidation: (14) C + H2O  CO + H2. C + 2H2O  CO2 + 2H2. (15). Water vapor can be introduced into the BF, with the burden moisture, hot blast moisture and due to a leaking cooling system. The water vapor oxidation of amorphous carbons starts at a lower temperature (450 oC) than for graphite materials (550 oC) [24,29,40]..

(27) Mechanisms of blast furnace hearth degradation. 27. 2.4. Deterioration by carbon monoxide Deterioration by carbon monoxide is a degradation mechanism that starts at 450550 oC in the presence of free iron and iron oxides. Carbon monoxide penetrates the open pores of the refractory materials and reduces Fe2O3 to Fe3O4 and further to free iron: 3Fe2O3 + CO  2Fe3O4 + CO2 (16) Fe3O4 + 4CO  3Fe + 4CO2 Then, the CO is adsorbed on the surface of the iron and a reaction occurs that results in the formation of FeC and free oxygen: (17) 2Fe + 2CO  2FeC + O2 The created FeC is then transformed into Fe2C and further into Fe3C. At each iron carbide transformation, free carbon in the form of soot is released: 2FeC  Fe2C + C (18) 3Fe2C  2Fe3C+C The overall interaction between the carbon monoxide and iron and iron oxides results in a phase transformation and soot deposition. Both these processes result in volume changes, which generates stresses and creates micro cracks. In order to reduce the deterioration by the carbon monoxide, the amount of the iron and iron oxides impurities in the refractory materials has to be maintain at a very low level [18,41–43].. 2.5. Slag attack Another degradation mechanism that occurs in the BF hearth is corrosion in contact with molten slag. The corrosion activity of molten slags affecting the refractory materials is dependent on their properties: viscosity, wetting capacity and surface tension, which depends on the slag composition [44,45]. Slag, which is a mixture of molten oxides (mainly CaO, SiO2, Al2O3 and MgO), is especially destructive to oxide refractory materials, which can dissolve in it. In order to prevent that, proper oxide materials have to be chosen, depending on the type of the slag, which can be basic or acid. The former has a higher amount of CaO and MgO over SiO2, while in the latter SiO2 is a major component. Therefore, in applications with basic slag, refractories based on MgO are used, while for acid slags, materials based on SiO 2 are used. Some materials are considered as neutral for both types of slag, e.g. MgCr2O4 or carbon. The interactions between various oxide materials and different slag compositions is a broad topic, going beyond the scope of this thesis and widely described in the literature, e.g. by Lee and Zhang [46] or Brosnan [47]. The carbon particles in the refractory materials can be oxidized by the reducible components from slag, such as FeOx, CrOx, MnO, SiO2. The reduction of iron oxides and chromium oxides is dominant over other oxides [45,48]. The reduction of iron oxides can be direct (19) by solid carbon or indirect (20) by carbon monoxide: FeO + C  Fe + CO (19) (20) FeO + CO  Fe + CO2 The process results in the formation of a liquid metal phase between the slag and the refractory material. However, carbon and the graphite materials are weekly wetted by the molten slag, as the initial contact angle between slag and graphite or coke is around 140o [49]. Thus, these materials are usually considered as resistant to molten slag corrosion. Moreover, carbon and graphite are often used as binders or particles in combination with oxides particles, e.g. in MgO-C materials, to improve their resistance to slag corrosion and thermal conductivity [46,50].. 2.6. Erosion and penetration by the molten metal The degradation of carbon refractory materials in contact with molten pig iron can be divided into two processes. The first one is erosion due to the molten metal flow, which is an example of mechanical.

(28) 28. 3D pore structure and infiltration resistance of micropore carbon materials. wear. The second one is a part of the chemical wear — the dissolution of the refractory material into molten metal. Wear might occur in contact with molten metal on the surface of refractory materials but also in their volume due to the fact that molten pig iron can penetrate their porosity. The penetration of a metal is limited to the location of the 1150 oC isotherm, i.e. the temperature below which carbon saturated pig iron cannot exist in the liquid form [51]. The erosion of carbon bricks starts with a contact between molten metal and a refractory material. The external surface of a refractory brick starts to dissolve in molten metal, while simultaneously molten pig iron penetrates the open pores close to the external surface. The infiltration and dissolution result in a strength decrease which leads to the embrittlement of the brick and its gets disintegrated into pieces. As a consequence, new open pores are accessible to infiltration by the molten metal and the erosion process is continued [52]. However, the processes of erosion and dissolution of carbon materials in molten pig iron are limited by a protective layer formed on the hot surface of carbon refractory materials. When the temperature of the hot surface of a refractory material is higher than the carbon saturation temperature Tc, the lining material will be dissolved by the molten metal. An increase in the carbon concentration leads to an increase in Tc. When the temperate of the hot surface is equal to Tc, the dissolution of the carbon brick stops. If the circulation of the hot metal is reduced or water cooling is enhanced, the temperature of the hot surface is further lowered, which leads to the formation of the protective layer called a skull [53]. The skull is made of solidified metal and slag, with phases such as CaZnSi 3O8, ZN2SiO4, CaMg2Al2O7, TiC, TiN, Ti(C,N) [53–55]. The created layer prevents the dissolution of the carbon refractory materials and its presence is considered as the main tool for prolonging the lifetime of the blast furnace hearth. It is a common practice to add titanium bearing compounds, e.g. ilmenite, to the BF burden, to reduce the viscosity of the molten metal and to precipitate the TiC and TiN particles. Both processes enhance the formation of the skull [56]. If there are discontinuities in the skull layer or even the whole layer disappears due to, for example, high molten metal velocities, the degradation of the carbon refractory materials continues. The erosion rate is affected by the properties of the molten metal, which depend on the temperature, its composition and the flow conditions. The erosion of the BF hearth lining can be controlled by the composition of the burden and reduced by the way of conducting the BF process, i.e. a low liquid level and the provision of suitable free space in the hearth will result in low velocities of molten metal. An important issue which has an influence on the erosion is the deadman, i.e. a pile of unburned inactive coke particles. Its position and movement affect the flow conditions, and thus might accelerate the erosion process in certain areas. As a result, three different wear profiles can be distinguished: the bowl type, elephant foot type and mumps face type [57,58]. Their schemes with a marked zone of the biggest erosion are presented in Fig. 2.1.. Fig. 2.1 Different examples of blast furnace hearth wear profiles: a) bowl type, b) elephant foot type and c) mumps face type [57].. When the protective layer of the skull is damaged, the molten pig iron can infiltrate the open pores in the carbon bricks. The penetration process depends on:  the surface tension of the molten metal;  the contact angle between the metal and the refractory materials;  the pressure of the molten bath column over the lining materials;.

(29) Mechanisms of blast furnace hearth degradation. 29. . the volume fraction of pores larger than 1-5 µm. The first three parameters can be altered by the operational conditions of the BF, while the forth one depends only on the quality of the chosen refractory materials. According to Silva et al. [32], the surface tension of the molten pig iron is about 600 mN/m and the contact angle between the molten metal and the carbon refractories is close to the 140 oC. Under such conditions, the 3-metre-high molten pig iron column cannot penetrate the pores smaller than 5 µm. These calculations have lead manufactures to the development of micropore and supermicropore carbon materials. One of the factors which affect the surface tension of the molten metal as well as the contact angle between the metal and the refractory materials is the composition of the molten metal. Deng et al. [58–60] investigated the influence of various elements present in the molten pig iron, such as: carbon, silicon, manganese, phosphorus, sulfur and titanium, on the erosion of carbon bricks. The results show that carbon, silicon and titanium decrease the erosion rate, while manganese, phosphorus and sulfur increase it. The influence of these elements on the erosion rate results from the fact that they increase (C, Si and Ti) or decrease (Mn, P and S) the viscosity of molten pig iron and change the surface tension, and thus the wettability, which for Mn, P and S might promote the mass transfer of carbon into molten metal. The authors suggest that content of those elements in molten metal should be below 0.04% for sulfur and above 0.10% for titanium. The degradation of carbon materials in contact with molten metal can be not only limited by changes in the molten metal composition but also by improvements in the refractory materials design. Erosion resistance depends on the type of carbon aggregate in the refractory material — aggregates with a lower graphitization degree and a higher ash content possess better resistance to erosion in contact with molten metal [61]. The penetration of molten metal into refractory materials can be limited by a reduction of material pore sizes by SiC whiskers, which are formed during the baking of micropore and supermicropore carbon materials. The dissolution of carbon brick into molten metal can be limited by the addition of alumina, which reduces the area of carbon components exposed to contact with molten metal. The TiC can also be added to materials to increase the viscosity of molten pig iron at the metal/refractory interface, which retards the hot metal flow and decreases erosion [25].. 2.7. Degradation of the blast furnace hearth All degradation processes described in the previous chapters have an influence on durability of the refractory materials, and thus the lifetime of the Blast Furnace. However, they do not occur separately as independent processes, but all contribute to the overall degradation of the BF hearth [20,62]. Based on the post mortem analysis of the BF hearth, Silva et al. [18] distinguish six layers between the hot and cold side, i.e. from the zone in contact with molten metal to the zone in contact with the cooling system. These layers were as follows:  lost layer — part of the carbon block eroded and dissolved by the molten metal;  protective layer — the skull layer with a lower thermal conductivity, deposited on the hot side;  hot metal penetrated layer — part of the carbon block whose pores were filled with metal;  brittle zone — embrittled layer of the carbon block;  slightly changed zone — part of the carbon block the physical and chemical properties of which have been slightly changed;  unchanged layer — the rest of the carbon block the properties of which are similar to the initially used materials. Based on the observed layers, the authors proposed a description of the overall mechanism of BF hearth degradation. In their description, four steps can be distinguish. They are presented in Fig. 2.2..

(30) 30. 3D pore structure and infiltration resistance of micropore carbon materials. Step 1 — Fig. 2.2a Molten metal penetrates the open porosity that is accessible from the hot side. This molten metal dissolves mainly the aggregates of the graphitic carbon, while the less graphitic carbonized binder is attacked on a lesser scale. The excess carbon dissolved from the lining can precipitate as graphite. The lining is progressively worn away by the flow of the molten metal. The hot metal can infiltrate pores up to the 1150 o C isotherm. After reaching this isotherm, the pig iron solidifies, which leads to volumetric shrinkage and the formation of cracks. The newly created voids can be further filled with the molten metal. Step 2 — Fig. 2.2b The erosion of the carbon bricks in the hot side affects the temperature distribution in the carbon block — the 1150 oC isotherm is shifted towards the cold side of the refractory materials. As a result, the molten pig iron can further infiltrate the carbon materials. Its shrinkage creates more cracks, which are accessible for metal infiltration.. Fig. 2.2 Four steps of the brittle zone formation described by the Silva et al. [18].. Step 3 — Fig. 2.2c Simultaneously with the hot pig iron infiltration, the steams of potassium, sodium and zinc penetrate the open porosity of the refractory materials. Their transport is limited to the 800 oC isotherm, and upon reaching it, they condensate and react with mineral phases (alumina, silica or aluminosilicates) or intercalate the carbon lamellae. The formation of the new phases is associated with the volume expansion, which induces the tensions and results to further cracks formation. The microstructure in this zone becomes disintegrated and crumby — it is often called the brittle zone. The changes in the microstructure decrease the heat transfer, since the temperature of the hot side is increased, which accelerates the degradation in contact with the flowing molten metal. Those two processes further shift the isotherms toward the cold side of the carbon blocks and increase the infiltration depth for the molten metal as well as for the zinc and alkali steams. Step 4 — Fig. 2.2d All of the presented mechanisms of the BF hearth lining degradation occur at temperatures above 500 oC, and thus the microstructure of the refractory materials behind this isotherm is unchanged. However, between the 500 oC isotherm and the brittle zone the microstructure of the carbon brick can be altered. This zone is known as the slightly changed zone. The changes are mainly a result of the deterioration by carbon monoxide — the process occurring in the presence of iron and iron oxides contaminations, which leads to the phase transformations and soot deposition and, in consequence, to the generation of cracks. However, compared to the molten metal and alkali and zinc steams infiltrations, the changes due to carbon monoxide deterioration are much less impactful..

(31) 3. Laboratory tests for carbon and graphite refractory materials degradation A good material design of the blast furnace hearth, or even the development of new materials that might extend the lifetime of the blast furnace, requires a comprehensive knowledge of the degradation mechanisms. One of the possible sources of such information is a post-mortem sample analysis, which provides valuable information about the working conditions and degradation mechanisms inside an actual BF hearth. However, this method is limited by the sample availability and provide only information about the sum of all degradation mechanism. The alternative which can be used to evaluate new materials and their resistance to various degradation mechanisms are laboratory tests. Such tests can be designed to simulate selected conditions inside an actual BF hearth. Laboratory tests are more available than post-mortem samples, less expensive and much quicker, which makes them a very important tool in the processes of designing and evaluating new refractory materials. Due to the conditions inside the BF hearth, most tests of these materials are focused on the interactions between solid refractory materials and molten metal or slag. The three main concepts of laboratory tests in such a system are presented in Fig. 3.1 — sessile drop, immersion and crucible tests [46].. Fig. 3.1 Schematic diagrams of common laboratory corrosion tests: a) sessile drop, b) immersion and c) crucible [46].. In a sessile drop test (Fig. 3.1a), a small amount of molten slag or metal, i.e. a drop, is placed on a substrate made of a tested refractory material. This setup allows investigating interactions between the molten corroding agent and the material, e.g. the dissolution and free infiltration, but also the wettability of a such a system by measuring the contact angle and surface tension. Despite the simple setup of the test, it can be modified in various directions, e.g. by opening the interface at high temperatures or by contact heating of both the metal/slag with the substrate instead of placing the molten corroding agent on the surface of the refractory material. The details of sessile drop measurements with possible modifications are presented in [63,64]. In an immersion test (Fig. 3.1b), one or more cylindrical or square pillar-shaped samples of the tested materials are submerged in the molten metal/slag. The molten corroding agents are held in the inert to them crucibles. The immersion tests can be performed under the static or dynamic conditions. The main difference between them is the motion of the samples, which more precisely simulates the working conditions of the refractory materials, and thus accelerates the degradation process. In a crucible test (Fig. 3.1c), the molten corroding agent is placed in the crucible made of the tested material. The method is simple and allows testing many samples in a relatively short time. However, it is static method and suffers from typical drawbacks of this type of tests, i.e. no temperature gradients, rapid saturation of the degradation products and no flow of the corroding agents. Except for the three briefly described types of tests, many modifications and more complex experimental setups can be found, e.g. rotatory slag tests or tests with inductions furnaces, which allow including the temperature gradients into the process or avoiding the saturation limits typical of immersion and crucible tests [46]. The current chapter will present the selected degradations tests, which are used to.