Fig.1.Base material pyramid with the location of the refractory materials.
Table 1.Refractory Consumption by the Steel Industry Worldwide
517
Journal of the Ceramic
Society of Japan 112[10]517 532(2004)
Carbon Based Refractories
J [{ ÜL ÏΨÌT v
Emad Mohamed M.EWAIS
Refractory &Ceramic Materials Lab.,Central Metallurgical R &D Institute (CMRDI ),P.O.BOX 87Helwan,11421Cairo,Egypt
Carbon based or containing refractories has been attracting great attention because of their unique high thermal conductivity,low thermal expansion,high resistance to thermal shock and
chemical inertness to the slag.They are classified into two groups;carbon /bricks /blocks and carbon containing materials.Carbon containing materials are further classified into carbon containing basic refractories and non­basic refractories.Manufacturing processes are considered.The physical,thermal,mechanical and chemical are reviewed.Antioxidant and bonding materials for these types of the refractory products are reviewed.Their appli­
cations are also considered.
[Received April 26,2004;Accepted July 30,2004]Key­words :Refractories,Carbon,Graphite,Magnesia,Dolomite,Alumina,Zirconia,Manufacturing,Antiox­
idants,Bonding,Physical,Thermal,Chemical,Mechanical,Application
1.Introduction
Refractories are materials (usually nonmetallic )that main­tain sufficient physical and chemical stability identity to be used for structural purposes in high temperature environments encountered in the process industries.While refractories are always exposed to high temperatures,the effect of other environmental conditions play a significant role in the perfor­mance of refractories during service.The
se include:mechani­cal stresses,thermal cycling and associated stresses,erosion and corrosion by hot gases and such molten materials as metals,slags,or glasses.Today,refractories are absolutely essential to industry.Without refractories,few manufacturing processes could be carried out.1)The production of metals,cement,glass,petroleum products,and much of our electrical energy depends on refractories.Yet,few people know relative­ly what refractories are or realize their importance,because these materials seldom come to the attention of the general public.Refractories are usually sent directly from their point of manufacture to another factory,where they are used to make consumer items that bear no trace of the refractories that were essential for their production.In addition to the category of refractories characterized by large­scale used in the process industries,there are others that are used for more specific applications.For example,in the aerospace industry extremely high temperatures are encountered from propulsion systems and friction heating at high velocities in the atmosphere.There are also applications in the field of nuclear energy.Such applications that may require the massive use of refractories are a vital factor in the success of a particular system.
Refractories are classified primarily on the basis of their chemical composition and the forms in which they are used.2) 8)To a lesser extent,refractories may be identified by association with a particular function such as thermal insula­tion,or a special manufacturing process such as fusion cast­i
ng.Although the types of refractories manufactured for the industrial use are vast,it should be recognized that only few chemical elements form refractory compounds are available in sufficient quantities to be used economically.The preponder­ance of all g heavy h refractories are manufactured from com­pounds that involve the elements:silicon,aluminum,magnesi­um,calcium,chromium,and zirconium.The oxides of these elements are singly used or in various combinations.Recently
and with increasing frequency,these elements are used in combination with carbon,Fig.1.9)Carbon or graphite refrac­tories are also used with carbon as the sole constituent in the form of blocks or bricks.
It is well known that the major refractories production is associated with the steel if steel production increases,refractories production also increases,and vice versa.10),11)Recently,a dramatic reduction of refractory consumption per ton of steel has been achieved in the iron and steelmaking industries.This is attributed to improve opera­tions specifically in process control and hot gunning repair.It also results from the development of continuous casting processes.The quality of refractories,specifically carbon containing or based refractories,truly advanced composite materials,also played an important role in achieving reduction in refractory consumptions (See Table 1).12),13)
Fig.2.Crystal structure of graphite.
518Carbon Based Refractories
In summary,there are many types of refractories.Carbon containing or based refractories are a class of refractories according to chemical composition classification.Because of the progress in this class and its application to iron and steel­making processes,the refractories containing or based upon carbon (carbon bricks /blocks,carbon containing basic refrac­tories,carbon containing non­basic refractories )are reviewed in the light of these processes.
2.Carbon and graphite refractories
Elemental carbon is found in nature in the form of dia­monds,graphite and above all as coal.For refractory pur­poses natural and artificial graphite (coke from coking plants )are important for the manufacture of carbon bricks.The rawmaterials for carbon blocks should have ash content as lowas possible as w ell as a high yield.14)
Because carbon or graphite refractories are both made up of the single element carbon,their distinction depends upon the basis of their crystal structure.Carbon refractories generally do not have a well­ordered crystalline structure and may be considered amorphous,depending on the initial rawmaterials and the temperature that reached during the manufacturing.The graphite structure is w
ell known,15) 17)Fig.2and indi­cates a planar structure with an infinite two dimensional array of carbon atoms arranged in hexagonal networks in the form of a giant aromatic molecule.The carbon­carbon bond (cova­lent )in the plane is strong as indicated by the interatomic distance of 0.142nm where the bonding (van der Waal type )between the planes is weak the interatomic planar spacing being 0.304nm.Consequently,graphite has a layered struc­ture and may occur in flakes or showpreferred orientation of its crystallites because of the alignment of the crystallites in fabrication he planar structure results has anisotropic properties.
The properties of graphite,in particular single crystal graphite,in terms of thermal expansion,thermal conductivity and compressibility are attributed to the structure.16),18)Thermal expansion perpendicular to the planes is 200times that parallel to the planes.Thermal conductivity paralleled to the planes is 200times that perpendicular to the planes.Its compressibility is 104 105times greater in the direction perpendicular to the plane.However,the degree of anisotropy decreases for the graphite components produced from a random array of graphite crystallites and the properties of such a body can't be readily inferred from orientation factors in the random structure.This is the case for manufactured carbon refractories as they contain amorphous carbon and /or well ordered crystalline structure.
Interesting properties for this refractory class in terms of
distribution of size pore,lowporosity,lowthermal expansion and zero permanent linear change at 1600 C,were reported,in particular,if this type is manufactured from anthracite cal­cined at very high temperatures (1600 1800 C ),extruded,im­pregnated.This improvement in the properties was based on the well distribution of the grain size and,in turn,minimizing the intergranular spacing of the product.19),20)
Three groups of refractory bricks out of carbon have been reported:amorphous,part­graphite or semi­graphite and graphite bricks.In recent years two further types designated as g microporous h were developed from the first two groups by additions of additives to improve their wear resistance.20)
2.1Manufacturing
Carbon and graphite refractories are naturally occurring graphite,coal,petroleum coke,coal­tar pitch,artificial graphite,coke from mines,gas­calcined anthracite and electri­cally calcined anthracite.21)Tar­pitches,petroleum pitches and other organic materials are used as binders.The raw materials are prepared,ground,sieved,classified,mixed to batches depending on the desired property values,heated to approximately 160 C depending on the type of bond,and
mixed w ith binder to get the so­called g green batch h or
g green mixture.h Next the mix is shaped.Vacuum vibration equip­ment,die presses,extrusion presses,isostatic presses and ramming equipment are used to form the batch /mixture into
the desired shape.After shaping the so­called g green h shape is
subjected to a firing process up to approx.1200 C.The binder converts to coke.It is possible to accomplish subsequent densification and compaction by impregnating the blocks with impregnation agents,which are similar to the binders with regard to their composition.The impregnation agent,which has entered the pores,is also converted to coke during further firing'.20) 22)
2.2Physico­chemical properties
Carbon and graphite are not wet by most molten materials because of the lowinterfacial tension betw een carbon or graphite and molten materials.They have excellent thermal shock resistance,and their strength increase when they are heated.At 2500 C,the tensile strength of graphite is roughly twice as great as its room temperature tensile strength,which is approximately 2000psi.1)Carbon and graphite have a range from good to superb electric conductivity,thermal conductivity and lowexpansion coeffic
ients.Their thermal shock resistance are sufficient for standard applications.Despite good thermal conductivity and thermal shock resis­tance the application of carbon blocks is restricted because they are susceptible to be attacked by oxygen,steam and CO 2in an oxidizing atmosphere above 400 C.
The wear mechanisms for carbon bricks lining the hearth and hearth wall of blast furnace were reported.23)Six wear mechanisms are responsible for the damage of carbon bricks;1)the dissolution of carbon in pig iron.24)2)the pick­up of potassium oxide and migration of it into the brick to tempera­ture zones of 900 C and reaction with the crystalline phases of ullite,a ­crystoballite and a ­quartz under the
formation of kalsilite (K 2O E Al 2O 3
E 2SiO 2)and leucite (K 2O E Al 2O 3E 4SiO 2).25)
These reactions are combined with a volume increase,which causes a destruction of the brick texture.Further pick­up of potassium and the formation of potassium­carbon compounds of the formula C 8K,C 24K and C 60K causes a swelling of the carbon bricks and a complete disintegration of it.3)MnO pick­up of the brick and its reaction with ash compounds at temperature above 1200 C forming manganese aluminium silicates.This in turn reduces the modulus of rupture of the brick.4)the
reaction of picked­up ZnO with
519 Emad Mohamed M.EWAIS Journal of the Ceramic Society of Japan112[10]2004
binding phases of the carbon bricks destroying it,formation of zinc orthosilicate(2ZnO E SiO2)or zinc aluminate during shut down of the blast furnace.25)5)oxidation of carbon by water vapor26),27)6)thermal stress due to the existing pressure within the lining.24)
2.3Applications
Because their properties,graphite and carbon refractories are serious contenders for applications in a reducing environ­ment.Blast furnaces use appreciable quantities of carbon and graphite,particularly in the hearth,but carbon and graphite may also find application in the bosh and other places such as tapholes.28)
With water­cooled shells,cupolas have been lined with carbon in the wells.Because of its high electrical conductivity, graphite is used for electrodes in electric furnaces to generates the arc.Because graphite can be easily machined,complicated shapes can be cut from stockin the form o bars,slabs,or cylinders.In order to accomplish the reduction process at electric arc temperature for th
e production of Si,FeSi, ,carbon electrodes with various graphite addi­tives are applied.In these furnaces the bottoms are partially lined with carbon blocks.
Carbon bricks are also installed as lining in tanks for making phosphoric acid because carbon has a very good resistance to acids.Wear resistant graphite plates are used as shaped bricks for the manufacture of fused cast corundum bricks.
It must be mentioned that there is no international standard for carbon and graphite refractories and properties depend only on the manufacturer according to the request of con­sumer.
3.Carbon containing materials
Graphite and carbon also are used in combination with other refractory materials to form a composite suitable for certain applications.The importance of carbon additions can be seen in the wear reduction by reducing infiltration depth and in the bond of the unfired bricks.In addition,thermal shockresistance is improved by increasing thermal conductivi­ty and decreasing thermal expansion.29)
The brickbond of unfired products in a cok ed operation state is based on the adhesion between the
coke lattice and refractory particles as well as on the adhesion within the coke lattice with partial direct atomic bonds,secondary valence bonds and van der Waal forces.
The infiltration depth is changed substantially by carbon from centimeters to millimeters.Consequently,the wear mechanism of the bricks is changed drastically.Two factors are responsible for this:
(1)The reduction of iron oxide in the liquid infiltration by metal.The eutectic temperature of the infiltrate is increased to its solidification point(with CaO/SiO2molar 2if in­filtrate contains FeO).
(2)Non­wetting between the oxide infiltrate and carbon of the brickat a contact angle f 90 (with CaO/SiO2molar  2if infiltrate doesn't contain FeO).
In addition to the FeO content,the CaO/SiO2molar ratio is a decisive in determining which of the two effects take place. The CaO/SiO2molar ratio regulates itself on the hot face side directly in the front of the carbon­containing brickzone.With magnesia and magnesia­doloma brick,the CaO/SiO2molar ratio always stays 2because of the high CaO content in the brick.Consequently,the reducing effect is always active with these bricks if the slags contain FeO.For magnesia bricks, however,this ratio can drop below 2as a result of the differ­
ent precipitation during infiltration.The iron ions of the infiltrate are absorbed by the periclase(MgO in accordance with the chemical balance within the solution).Consequently, the melt losses iron before it reaches the carbon in the brick. Due to the CaO/SiO2molar ratio dropping 2,the infiltrate remains liquid and agile despite its loss of iron.In this case, the non­wetting effect prevails.The same applies for slags without FeO.
The decisive factor for effective utilization and application of C­containing bricks is the burnout speed of carbon.In pitch or resin­bonded bricks,a cracked carbon lattice forms the brickbond.This means that carbon burnout leads to a decisive bond loss.Consequently,C­containing bricks can and should be used where reducing gases,that is a furnace atmosphere with low oxygen partial pressure,are predo­minant.Examples are converters,electric arc furnaces or the metallurgical ladle.In order to lower speed,additives for retarding oxidation are also used.
Carbon containing materials are divided into carbon based basic refractories and carbon based non basic refractories. Each type also can be classified according to the binders used. This means that the carbon to refractories carry out via differ­ent techniques to manufacture these types.
3.1Manufacturing
The brick s or the block s of these types of refractories are manufactured by grinding the raw materials,sieving,classifi­cation,mixing to batches depending on the desired proper­ty values,heating to approximately100 200 C depending on the type of bond,and mixing with binder to get the so­called g green batch h or g green mixture.h However,the bricks bonded with synthetic resin are manufactured cold or hot 100 C with liquid and a hardener.Next the mix is shaped. Vacuum vibration equipment,die presses,extrusion presses, isostatic presses,hot pressing and tempering,and ramming equipment are used to form the batch/mixture into the desired shape.After shaping,the so­called g green h shape is subjected to a firing process up to approx.1200 C.The binder converts to coke.It is possible to accomplish subsequent densification and compaction.It is possible to accomplish subsequent den­sification and compaction by impregnating the blocks with impregnation agents,which are similar to the binder in regard to their compaction.The impregnation agent,which has entered the pores,is also converted to coke during further firing cycle.
3.2Bonding materials
The binding materials for carbon based refractory materials should have an ash content as low as possible as well as a high yield.Tars,coal­tars or coal­tar pitches are the commonest types of materials used as carbon sources and binders for refractory bricks.They have long been used in practice.The
physical properties of the tar or pitch influence the processing behavior greatly during manufacture.Conversely,the choice of binder is also determined by the particular processes in use.30)The softening point of the residual binder should not be exceeded prior to carbonization for any given bricks to prevent spalling.31)
Because of the potential health hazards in the handling of such materials and the evolution of hazardouds pyrolysis products,there is a tendency to use polymers to replace tars, coal tars and coal­tar pitches.32) 35)Phenolic resins,both novalacks and resols are favored because they are or can become thermosetting and because they can be pyrolysed during coking to achieve a high carbon yield.In addition to favorable pyrolysis and carbonization behavior,they are available in various forms,such as solutions,powder resins,
520Carbon Based Refractories
solid materials,and melts.They also serve as binders and impregnating agents for carbonaceous materials and refracto­ry products.Synthetic resin bonded bricks offer the following basic advantages:36),37)
(1)Their production and processing is environmentally acceptable.
(2)Their production by means of the cold mixing method conserve energy.
(3)The products can be processed in uncured conditions.
(4)The products have no plastic phase when heated up,in contrast to tar­pitch binders.
(5)The carbon content(more graphite or soot)can increased to augment resistance to abrasion and slag attack. The control in cure of the resins is very important factor where some resins tend to harden in a comparatively short time.This,in turn,reduces the time that mix can be retained before it must be shaped into bricks or other desired shapes. Lyer and Shah38)contributed in solving this problem by control in pH in the presence of a catalyst.
During heating of this product in the operation state,the pitch form elementary carbon(coking)at300 600 C by pyrolysis in liquid state leading to a separation of the volatile constituents(cracking).29)Laminated carbon packages form with good structure at the hexagonal base plane.These carbon packages are easy to graphitize and possess good optical anisotropy.However,in the c direction the carbon planes stay shifted and twisted,with varying degree of distance,even at the highest operation temperature.This intense degree of one­dimensional imperfection corresponds to a polycrystalline graphite lattice(the three­dimensional graphite structure
is not obtainable until 2000 C).In contrast to pitch,the pyrolysis of phenolic resins,which are synthetic polyconden­sation products occur in solid state.The strong space inter­lacement of the molecule chains and,as consequence,missing agility prevents an oriented deposit of hexagonal carbon layers.A poorly arranged and strongly interlaced lattic is the result.This prevents gliding and cleavage.Therefore,these cokes are hard and very sensitive to oxidation due to the large inner surface.39)
Phenolic resin has been widely used for its excellence knead­ing properties,molding properties and economy for carbon containing bricks.Although this binder has superior proper­ties but it has some drawbacks.Firstly,a phenolic resin gener­ates gases such as water,hydrogen,ethylene,phenol,cresol and xylenol when carbonized in temperature range of from 350 650 C causing air pollution and odor,etc.40) 42)Secondly, where a phenolic resin is used as a binder,the resulting struc­ture is dense and has insufficient open cells.Therefore,the structure is liable to destruction due to the evolution of decomposition gas on heating.Thirdly,carbon produced from a phenolic resin is a glassy carbon inferior in resistance to spalling.Although the phenolic resin has a high residual car­bon on burning,the resulting products have poor spalling resistance.Yamamura et al.43)proposed the using saccharified starch of a non­aromatic organic high­molecular compound to overcome the disadvantag
es of the pitch­bonded type and using of a hexahydric alcohol as well as the first to the third drawbacks associating the use of a phenolic resin.
3.3Carbon­containing basic materials
3.3.1Carbon containing magnesia refractories
The evolution and use of magnesia refractories in combina­tion with carbon started over forty years ago,in the early 1950's,with pitch bonded dolomite refractories,developed primarily for the basic oxygen furnace.In these early days, some of these linings in the basic oxygen furnace lasted only 100heats,often not giving sufficient time to reline the se­cond vessel in a two furnace shop.44)
Very measurable improvement came when magnesia fines were used in conjunction with the dolomite coarse fractions bonded with pitch.Further improvements came with the all magnesia pitch bonded brick.In the1970's the burned and impregna ted ma gnesia brickwith finite pore size beca me the standard for the charge pad and other high wear areas,start­ing the beginning of the zoned lining for basic oxygen furnaces.45)About that time magnesia purity became a factor and a special low boron96÷magnesia grain having a lime to silica ratio of2to3F1was used extensively.
The1980's saw the development of resin bonded magnesia­graphite,first with higher carbon content and then with the addition of antioxidants to preserve the carbon content.46),47) Recently fused magnesia grain,sintered magnesia with larger crystallite size,and very high purity sintered magnesia were introduced to further improve the corrosion resistance.48) In addition to conventional pitch and resin bonded and burned and impregnated magnesia brick,the following three types of ma gnesia ca rbon bricka re a va ila ble on the ma rk et.«The first series contains regular sintered magnesia(97÷Mg0)with medium quality graphite(95÷C).
«The second contains high purity sintered magnesia(99÷Mg0)with high purity graphite(99÷C).
«The third contains high purity sintered magnesia with high purity graphite plus antioxidants.
Magnesia carbon refractories are classified into three types according to the carbon:
(1)Fired carbon­containing magnesia bricks( 2÷C),
(2)Carbon­bonded magnesia bricks( 7÷C)49)and
(3)Carbon­bonded magnesia­carbon bricks( 7÷C)50). The third one of refractory type is best suited classify on the light of binder to two types as following:
(1)Bricks with pitch bonded(with max.15÷C),partial­ly with subsequent pitch impregnation under vacuum or secondary for special cases with antioxidants,and,
(2)Brick s with synthetic resin bond up to25÷C)often with added antioxidants(Al,Si,AlSi,Mg,AlMg,B4C among others in powder form).
3.3.1.1Production technology
MgO C bricks are manufactured from high purity MgO atural,sea water and electrofused magnesia as well as flaky graphite with carbon content of86 99÷,carbon bla ckwhether therma l or furna ce with na tura l ga s or oil base,and coke binders whether pitch or synthetic resins for unfired products.In these types,a high chemo­thermal stabili­ty is required from magnesia in bricks containing carbon.This means that a CaO/SiO2molar ratio of magnesia should be 2 as well as possibly slight contents of B2O3and SiO2which form melts;furthermore,little Al2O3and Fe2O3.In addition, periclase boundary surface should be a negligibly reactive due to low apparent porosity and large crystals.51),52)
Fired carbon­containing magnesia bricks are manufactured by placing the fired magnesia bricks under vacuum at150 200 C and impregnated with pitch or resins.In this process,it is not possible to o
btain more than2÷residual carbon. Repeated impregnation along with intermediate coking or the separation of carbon out of furfuryl alcohol,for example,to increase the carbon content,have not proven to be economi­cally feasible.
Concerning to the properties,these bricks only differ from fired magnesia bricks in regard to infiltration­retarding effect of carbon.This has a decisive influence on the wear behavior but is hardly noticed when comparing test or inspection value.
Table
2.Properties of Carbon Containing Magnesia and Magnesia Carbon Bricks
Table
3.Thermo Mechanical Properties of Carbon Containing Magnesia and Magnesia Carbon Bricks
521
Emad Mohamed M.EWAIS Journal of the Ceramic Society of Japan 112[10]2004The bricks are distin
guished by superb erosion resistance due to the firing of the brick.However,the thermal shock resistance is only moderate (See Tables 2and 3).
Carbon­bonded magnesia brick (pitch impregnated )is manufactured by mixing magnesia and pitch at 100 200 C,these bricks are shaped while still warm and tempered at 250 350 C to obtain sufficient hot strength.In addition to tempering and using pitch instead of tar for improving the carbon bond,carbon black is added,dehydrogenation agents are used and pitches impregnates under vacuum are further step.Carbon black can be dispersed in pitch in such way that it will greatly strengthen the coke bond.Dehydrogenation agents such as sulfur also increase the output of carbon.
N.B.1 Hard coal pitch has the best performance /cost relative
of all coked binders.
2 Phenolic resin is not only used as binder for magnesia­carbon bricks
with  7÷C but also sometimes for magnesia bricks above 5÷C.
Carbon­bonded magnesia carbon bricks are characterized by residual carbon content of more than 7÷.The obtained high content of residual carbon can't be reached even by addi­tion of graphite.
Carbon­bonded magnesia carbon bricks with pitch bonded are manufactured by hot pressing and tempering at 250 350 C.The bricks bonded with synthetic resin are manufac­tured cold or hot at temperature less than 100 C with liquid phenolic resol or phenolic novalac solutions and a hardener.Next,the binders are hardened at 120 200 C.Due to interlac­ing,this ensures formation of a highly molecular and non­meltable resite lattice.
ignore subsequent bad blocksCarbon­bonded magnesia carbon bricks having high pro­perties in respect of resistance to thermal,structural spalling,slag resistance and thermal shock resistance,etc.It is manufactured from 60 97÷sintered magnesia of bulk specific gravity about 3.4and 3 40÷carbonaceous materials and a carbon bonding formative agent 53)for applications in the converter.
The effect of impurities in magnesia upon the reaction between magnesia clinker and carbon was estimated by meas­uring the weight loss of MgO specimen reacted with graphite at 1500 1750 C.54),55)It was found that the reaction between the MgO clinker and the carbon was affected by the chemical composition of the clinker rather than by its crystal size.

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