Lakhasly

CHAPTER I 1- INTRODUCTION

1.1- Refractories

Refractories are traditional ceramic materials that can afford high-temperatures without deterioration .Table 4.Most common additives in refractory castables Function Additive Accelerator Retarder pH control Water Reducer Rheology Modifier Lithium carbonate X Calcium hydroxide X X Sodium carbonate X X Sodium bicarbonate X Sodium citrate X X X Sodium phosphate X X X Sodium polyacrylate X X Polycarboxylate X X Citric acid X Boric acid XRefractory brick Refractory monolithic Uniformity More uniform because they do not contain cement and fired at high temperatures with out using anchors [Innovations reference] Less uniform and is used by using anchors Flexibility More flexible due to more lining joints Less flexible Ease of use Labor intensive which Requires time and highly skilled technicians Simple and less time consuming but carful curing is required Pore size Pore size ranges from 20-25 microns Micro porous structure with pore diameter 1-2 um Strength and thermal shock resistance Low strength and TSR due to relatively large pore diameter size High strength and TSR due to small pore diameter Thermal conductivity Higher thermal conductivity Lower thermal conductivity due to lower radiation heat transfer through small pore size Corrosion resistance Low corrosion resistance Higher corrosion resistance Deformability Less flexible More flexible especially phosphate-bonded castables Dimensional stability More stable as it is a fired product Less stable due to shrinkage during firing in the installation place

1.2.2 Refractory Castables

     Monolithic materials were first used in 1914 when as simple mixture of crushed firebrick and fireclay was produced (Shubin, et al., 2001).Common modifiers used in refractry castables

Filler/Modifier Chemical formula Function Fine milled aggregates Various Chemistry, Mineralogy and adjustment, bond modification and development Calcined alumina ?- Al2O3 Chemistry adjustment, bond modification and development Reactive alumina ?- Al2O3 Flow/rheology control, bond modification and development Silica quartz SiO2 Shrinkage control Kyanite 3 Al2O3.3SiO2 Shrinkage control (1325-1410 C), chemistry and mineralogy adjustment Clay Hydrated alumino-silicate Filler, flow/rheology control Zircon ZrSiO4 Reduce metal, slag, attack Graphite/Carbon C Reduce metal, slag attack Graphite/Carbon C Reduce metal, slag attack Fly ash Varies Low-temperature filler

The properties of the castables which includes expansion control, bond enhancement, and mineralogy/chemistry adjustment can be controlled by using additives or admixtures in small amounts ranging from 0.05-0.5 wt. % (Bartha, et al., 1999) .The different mineralogical forms (individually or in combination) of oxides such as alumina (Al2O3), calcium oxide (CaO), silica (SiO2), magnesium oxide (MgO), zirconium dioxide (ZrO2), chromium oxide (Cr2O3), and carbon (C) are the most suitable and widely used materials in the refractories applications (Sengupta, 2020).Some of these materials are unsuitable to be used as refractories on the industrial scale due to their fast reaction with atmospheric moisture, such as calcium carbide (CaC2), barium oxide (BaO), and aluminum carbide (Al4C3) or their high costs such as molybdenum (Mo), niobium (Nb), vanadium (V), and hafnium (Hf).e. Corundum refractories: it consists from a single phase (polycrystalline alpha alumina) with alumina more than 99 wt.% f. Magnesite refractories: they are basic in nature with magnesium oxide as a basic constituent at least 85 wt. % g. Dolomite refractories: It is a double carbonate mineral of calcium and magnesium when fired transferred to oxides, usually contains less than 2.5 impurities and greater than 97.7% (CaO + MgO).White fused alumina (WFA) is manufactured by the fusion process of Calcined alumina in an electric arc furnace and its microstructure is characterized by corundum grains while brown fused alumina (BFA) is produced by bauxite fusion with higher impurities compared to WFA.Cement free castables then developed with further higher refractoriness due to the very low calcium oxide where they have superior corrosion resistance towards metals and slags but have lower physical and mechanical properties compared to LCCs and ULCCs (Javed, et al., 2004) .Table 2.Refractory Castables composition Aggregates 40-80 % Modifiers 5-30 % Bond agents 2-50 % Admixtures <1%

The aggregate Skelton is filled by finer particles which is responsible mainly for rheology and flow characteristics while the structure is held together by using a binder or bonding agent.Refractory castables can be classified according to its lime (CaO) content (Shubin, et al., 2001) into conventional (CaO > 2.5 %), low cement (LCC) where 2.5 > CaO >1.0, ultra-low cement (ULLC) where 1.0 % > CaO > 0.2%, and no cement castables (NCC) where CaO <0.20 (Eden, et al., 2013) .Also, it may be classified according to installation method where it can form precast shapes or poured directly into the installation site, casted into place, vibrated, troweled, or projected (spraying or shotcreting) (Javed, et al., 2004).The significant water part in conventional castables (6-10 %) reacts with the cement to form hydration products while the remainder (2-6%) aids for proper installation and did not take part in the reactions so the porosity of the castables is enlarged (Bartha, et al., 1999) .Refractory materials are inorganic solids in the form of oxides, carbides, nitrides, and borides of aluminum, silicon, alkaline earth metals, and transition metals.This reaction can be speeded by alkali metal salts addition or slowed down by carboxylic acids (Rodriguez, et al., 2004) and the silica could be added to forms mullite or 0.5 wt. % of cement is used with hydraulic alumina to control the setting time.1-3 Refractory Castables composition

Castables are composed of refractory aggregates with different sizes from 20mm to 300um for strength and hot properties beside matrix components, bonding agents and admixtures (Bartha, et al., 1999) as shown in table 1 .Refractories are stable at elevated temperatures (high thermal shock-resistance), stand firm against corrosive solids, liquids, and gases (chemical resistance), and can maintain their structural strength as well as physical shapes at elevated temperatures (Sengupta, 2020).Generally, the corrosion resistance of refractories depends on some factors such as the texture of the sintered body as well as, relative density/porosity, solubility limit, and microstructure of refractories.Low cement and ultralow cement castables were then developed to decrease the lime content by using smaller amounts of cement because CaO spoils the high temperature characteristics of castables (Shubin, et al., 2001) .The corrosion of refractories occurs as a result of different mechanisms combination such as dissolution, oxidation-reduction reactions, structural/chemical spalling, and penetration.1.2.3.1.2.1.

CHAPTER I
1- INTRODUCTION

1.1- Refractories

Refractories are traditional ceramic materials that can afford high-temperatures without deterioration . Refractory materials are inorganic solids in the form of oxides, carbides, nitrides, and borides of aluminum, silicon, alkaline earth metals, and transition metals. Refractories are stable at elevated temperatures (high thermal shock-resistance), stand firm against corrosive solids, liquids, and gases (chemical resistance), and can maintain their structural strength as well as physical shapes at elevated temperatures (Sengupta, 2020).
Generally, the corrosion resistance of refractories depends on some factors such as the texture of the sintered body as well as, relative density/porosity, solubility limit, and microstructure of refractories. The corrosion of refractories occurs as a result of different mechanisms combination such as dissolution, oxidation-reduction reactions, structural/chemical spalling, and penetration. Furthermore, grain boundary corrosion, stress corrosion, mass transportation, diffusion, absorption, and desorption are all part of these mechanisms (Nath et al., 2016).
Refractories should be stable under normal atmospheric conditions, available in high quantities, and low cost to be successfully used on the industrial scale. Some of these materials are unsuitable to be used as refractories on the industrial scale due to their fast reaction with atmospheric moisture, such as calcium carbide (CaC2), barium oxide (BaO), and aluminum carbide (Al4C3) or their high costs such as molybdenum (Mo), niobium (Nb), vanadium (V), and hafnium (Hf). The different mineralogical
forms (individually or in combination) of oxides such as alumina (Al2O3), calcium oxide (CaO), silica (SiO2), magnesium oxide (MgO), zirconium dioxide (ZrO2), chromium oxide (Cr2O3), and carbon (C) are the most suitable and widely used materials in the refractories applications (Sengupta, 2020).
Refractories are designed to be used in some applications such as heat- resistant walls, linings, or coatings to protect structures from oxidation, corrosion, and heat damage. Castables, bricks, ceramic fibers, and plastic refractories are the main types of refractories (Sadeghbeigi, 2020).
1.2 Refractories classification
Refractories can divided according to different bases like chemical structure, chemical behavior, or in terms of physical state.

1. 
   

   According to chemical structure: they are classified according to the major constituents that forms the refractory for example
   a. Silica brick: a refractory that contains at least 93 wt. % silica.
   b. Fireclay brick: its phase diagram lie in the system SiO2-Al2O3 with silica content up to 78 percent and alumina amount up to 44 percent.
   c. High alumina refractories: it is alumina silicate refractories with a dominant alumina phase reaching more than 45 percent.
   d. Mullite refractories: A refractory structures which is consisted from alumina (71.8 wt. %) and silica (28.2 wt. %) silica .
   e. Corundum refractories: it consists from a single phase (polycrystalline alpha alumina) with alumina more than 99 wt.%
   f. Magnesite refractories: they are basic in nature with magnesium oxide as a basic constituent at least 85 wt. %
   g. Dolomite refractories: It is a double carbonate mineral of calcium and magnesium when fired transferred to oxides, usually contains less than 2.5 impurities and greater than 97.7% (CaO + MgO).

   
   


2. 
   

   According to chemical behavior: They are classified according to their basic or acidic behavior towards chemical attack
   a. Acidic refractories: they are chemically noble towards acidic compounds but are attacked by alkalis eg. Silica (Bray et al., 1985).
   b. Basic refractories: they are non reactive towards basic compounds but are attacked by acidic compounds e.g. magnesite and dolomite.
   c. Special refractories: silicone carbide, zirconia, and carbon refractories.

   
   


3. 
   

   According to physical state: They are classified according to physical form when they are used whether they are pre-shaped refractories like bricks or loose powder called monolithic (Bray et al., 1985).
   1.2.1. Shaped refractory versus refractory monolithic

   
   

   Refractory material could be precast with a fixed shape that is fired prior to installation which is called refractory brick or it may consist of different material with different particle sizes that are applied to the installation place to fill where it is dried and fired which is called monolithic. Installation may be done by casting or pouring or through trowelling, vibration, spraying or shotcreting as shown in Figure. A comparison made between refractory brick and monolithic is illustrated in Table 1. 
   
   

   
   

Figure 1. Wet gunning of bauxite based castables (Bartha, et al., 1999)
Table 1.A comparison between shaped refractories (Bricks) and unshaped refractories (monolithics).
Refractory brick Refractory monolithic
Uniformity More uniform because they do not contain cement and fired at high temperatures with out using anchors [Innovations reference] Less uniform and is used by using anchors
Flexibility More flexible due to more lining joints Less flexible
Ease of use Labor intensive which Requires time and highly skilled technicians Simple and less time consuming but carful curing is required
Pore size Pore size ranges from 20-25 microns Micro porous structure with pore diameter 1-2 um
Strength and thermal shock resistance Low strength and TSR due to relatively large pore diameter size High strength and TSR due to small pore diameter
Thermal conductivity Higher thermal conductivity Lower thermal conductivity due to lower radiation heat transfer through small pore size
Corrosion resistance Low corrosion resistance Higher corrosion resistance
Deformability Less flexible More flexible especially phosphate-bonded castables
Dimensional stability More stable as it is a fired product Less stable due to shrinkage during firing in the installation place

1.2.2 Refractory Castables

     Monolithic materials were first used in 1914 when as simple mixture of crushed firebrick and fireclay was produced (Shubin, et al., 2001).  By the 1960s, high purity calcium aluminate cement based castables was common (Javed, et al., 2004) . From 1970, the manufactures started developing low cement castables to improve the hot properties (Gerald Routschka, et al., 2005).  The rapid growth in using monolithics and its related products began since 1980 due to rapid, easier and cost installation process and less lining joints (Bartha, et al., 1999) . 

      Castables are important from the economic point of view where they form not less 40 % of the refractories market share (Javed, et al., 2004)  . Alumina and alumina silicate based castables are commonly used while basic castables accounts for 20 % sales of the castables in the market (Chen, et al., 2007).   



   Conventional castables usually contains 15-30 % CAC so larger amount of water is added (8-15 wt. %) to gain sufficient strength while up to 5.0 wt. % water is often filling the pores compared to low and ultralow cement castables (Philip, et al., 2014) . Refractory castables can be classified according to its lime (CaO) content  (Shubin, et al., 2001)  into conventional (CaO > 2.5 %), low cement (LCC) where 2.5 > CaO >1.0, ultra-low cement (ULLC) where 1.0 % > CaO > 0.2%, and no cement castables (NCC) where CaO