Use of Graphite in Refractory: Burning or using

 Graphite is a natural limited resources. Because of its unique properties like flaky nature, non wettability with liquid metal and high thermal conductivity along the axis, used in Refractory. Anti oxidant is used along with graphite during manufacturing of refractory to take care of oxidation. But a portion of graphite is burned in the furnace, which can be avoided.

Graphite or Carbon is used largely together with silicon carbide in ramming masses and castables. The function of carbon is prevent premature oxidation of silicon carbide in these refractory products. Depending the time and temperature the carbon is oxidized resting only the SiC to protect the product . If SiC is also oxided then we will have erosoin or ling wear. In consequency the life will be shorted.

Natural graphite consisting of clay material is utilized for the production of refractory blocks, crucibles,sheaths,high temperature lubricant and it is also used as a lining material for ramming the tapholes,and the colloidal graphite is used as mould release compound in foundary practice. Besides which it is also used as reductant for the reduction of metal oxides to produce metals which means it is useful during burning also. In recent years there have been efforts to increase the oxidation resistance of graphite by the addition of anti-oxidants to the surface graphite particles. As against the natural graphite the synthetic and pyrolytic graphite are very pure and highly crystalline substances and because of their outstanding high temperature thermal, electrical conductivity and high thermal strength they are used used in specialised applications such as electrical commutation, spectral elecrodes, heating element,high purity crucibles,thermal seals, metal matrix composites.  

Improvement of refractory life and performance

Improve your Refractory Life and Performance by following 3 steps

Nothing counts like the 'performance'. Getting or giving a better performance is one thing which everyone tries to do. Reasons obvious! For getting better performance of refractories from an installation (lining) i.e. an improved refractory life, one must take care of the following three simple but very important things:

1. Proper Selection of Refractories.
2. Proper Installation - Laying of Refractory Bricks.
3. Proper Operation Practice.

Selection of Refractories

Though there are specific refractories for different applications, operation practices lead to certain criteria on which, depends the refractory life. As such these need to be properly considered. Customers should disclose the actual operating practices and conditions so that some important properties, required to such conditions can be taken care during the selection of refractories as well as during the manufacturing stage of such refractory bricks, castables, and mortars. Among various physical, chemical, thermo-chemical and thermo-mechanical properties of Refractories, there are a few properties which can change significantly performance of refractories. These are called Key Properties. For ensuring better performance, quality and of course, refractory life these key properties should be tested.

Installation - Laying of Refractory Bricks

Depending on the method of application or installation there has to be a set of guidelines in respect to laying of refractory bricks, their dimensions, selection of mortars, expansion joints and many other minute but very important things. So from case to case basis the supplier of refractories should specify this properly and also ensure that the methods are actually being followed. Transportation, handling, timely arrival of refractory bricks, mortars, skill of masonry work, proper equipment for application e.g. mixer machine for castable, vibrator for installation, forma etc. are very important. Maintaining proper expansion gaps, correct dimension (size) of bricks and monolithics, fixing anchors etc. all are very important to achieve better life of refractories. Once the laying job and other installation of refractories are over, the initial heating of the lining before starting the actual operation is of prime importance. Customers should demand the initial heating schedule from the refractory supplier.

Operational Practices

Proper operation is not only important for getting right quality output but also, help in getting the optimum refractory life, less downtime, maximum availability of the furnace and thus, the benefit of lower cost of refractories per tone of finished product. Customers must be aware of the reasons which can damage the refractories arising because of improper operations. During the training of the furnace operators, apart from the method of the furnace operation etc. they must be given some knowledge regarding the proper usage and importance of refractories also.

Benefits of using Steel Fibers and Organic Fibers in Refractory Castables and Monolithics

One of the most effective ways of improving the mechanical and thermal properties of refractory castables and other monolithic refractories is adding in suitable proportions of stainless steel fibers and organic fibers to the castable respectively.

Steel Fibers

Steel fiber reinforced refractory castables are very resistant to the tendency of the material to fall apart on thermal cycling. Stainless steel fibers greatly improve the flexural strength of the castable. And this added increase in ductility contributes significantly to the thermal shock and spalling resistance of the material. The fibers generally used are in size varying between 0.1 to 0.4 mm2 in cross-section & 20-40 mm in length. For monolithic SS is used either high chrome or high chrome nickel steels available in the market with different grades. One reason commonly reported that the thermal shock resistance of castables is greatly increased through addition of SS fibers because these fibers act as crack arresters, preventing cracks propagating. This is also possible that the microcracks caused by a mismatch in thermal expansion coefficients of matrix and fibers dissipate energy from larger cracks propagating as a result of thermal stress. However percentage of these fibers added becomes important because of two reasons as it has a direct impact on the fluidity of the castable, then it may also cause mixing difficult due to fiber-balling when added beyond 3% by volume. Another critical factor will be the maximum application temperature for the castable that those fibers present in the castable can resist oxidation (since these fibers can not perform beyond their melting temperature).

Organic Fibers

An effective means for improving the explosive spalling resistance of a castable is to add organic fibers to the formulation. It has been reported that the composition & concentration of fibers are not as important as melting temperature of the fiber, since these fibers after melting increase permeability at certain temp. & thereby reducing the explosive spalling tendency of the castables. The fibers generally used for this purpose are Polypropylene fibers, Polyester staple fibers, etc.
Because of these different advantages it have been found that both organic and SS fiber reinforced refractory castables provide substantial increase in service life and therefore, a considerable reduction in refractory maintenance cost and furnace down-time.

Calculation of Ideal Mold Taper

The narrow-face of the mold should be tapered to match the shrinkage of steel shell, which is cooling against the wide face.Shrinkage depends mainly
on the surface temperature of the shell and the steel grade. The model predicts ideal average taper, by dividing the thermal strain, ε, by distance down the mold (instantaneous taper) or by the mold length (total taper per m). Thermal shrinkage strain is estimated here εth1, by:
ε th1 = TLE(Tsol ) −TLE(Ts )

Here, TLE is the thermal linear expansion function for the given steel grade,Tsol : solidus temp.(1509), Ts : shell surface temp.

TLE(Tsol) =α (T −Tsol ) , α is thermal linear expansion co-efficient

calculated from
weighted averages of the phases present.
For the sample cases, the higher speed causes a hotter shell with less shrinkage, , so needs slightly less narrow face mold taper. The shrinkage εth1, based on surface
temperature only and is almost independent of casting speed, due to the cancellation effect. With a linear taper, the narrow-face shell attempts to shrink away from the upper portion of the mold, while it pushes against the lower
portion of the mold. To match the shrinkage, it is clear that taper should be increased high in the
mold and decreased lower down. Mold distortion, viscoplastic creep of the steel, and other factors should also be taken into account when designing a non-linear mold taper. These calculations require sophisticated thermal-stress models, to calculate temperatures, stresses, and shrinkage, including the formation of an air gap near the corners, and its effect on heat flow across the mold/shell interface. The calibrated CON1D model is currently being used to provide calibrated heat transfer data to these models to evaluate and improve taper optimization.

Reduction of Slivers DueTo Nonmetallic Inclusions inContinuous Casting

This is one of the major defects observed in steel slabs that appear as an extra layer on the surface of cast slabs.The generation of slivers is mainly due to the nonmetallic inclusion(NMI) of the liquid steel. The sources of generation of these NMI start from EAF tapping and continue till Continuous casting in different sections. The
slivers are divided in to two types: one is FeO and the second is Al2O3.

Tunnel furnace may also contribute slivers for high percentage of oxygen percentage generating scales which may some time not removed in high pressure De-scalar which later appears like slivers after rolling.

During Tapping:

The Al2O3 is primarily generated duringtapping from EAF to deoxidize the oxygen present in the steel bath. During this period, the O 2 pick up takes place due to its exposure to atmosphere generating more Al2O3. Generally, 180 tons liquid steel takes around 3-5 minutesto complete its tapping depending upon its EBT life.

Figure  shows the chemical reaction that occurs inside the ladle furnace,

formation of calcium aluminate and its effect in slabs. The formations of CaO.Al2O3 (CA) complex

compounds and propertiesare shown below table no 1:

Table no 1: Chemical reactions in ladle furnace
Reactions [12] Compound (C-CaO,A-Al2O3)
Melting point, °C Density, gm/cc
3CaS +19Al2O3 = 3(CaO.6Al2O3) + 2Al + 3S C6A 1833 3.38
12CaS + 7(CaO.6Al2O3) = 19(CaO.2Al2O3) +
8Al + 12SC2A 1755 2.91
3CaS + 4(CaO.2Al2O3) = 7(CaO.Al2O3) + 2Al
+ 3SCA1590 2.88
15CaS + 33(CaO.Al2O3) = 4(12CaO.7Al2O3) +
10Al + 15S C12A7 1395 2.83

Effect of phosphorus reversion:

The electric arc furnace slag has high contents of FeO and MnO. Itis well-known that high levels of those oxides produce a harmful effect on steel cleanliness, bringing about an increase in the total oxygen content of the steel. FeO and MnO in Slag- An important source of reoxidation is the carryover slag from the EAF to the ladle, which contain a high content of FeO and MnO. These oxides react with the dissolved aluminum to generate alumina in liquid steel, owing to the strong favorable thermodynamics of the following reactions [1]:

3FeO (l) +2Al =Al2O3 +3Fe (l)?Go = -853700+239.9T (J mol - 1) (1)
3MnO +2Al =Al2O3 +3Mn (l) ?Go = -337700+1.4T (J mol - 1) (2)

The higher the FeO and MnO content in the ladle slag, the greater is the potential for reoxidation and the corresponding generation of alumina inclusions. Many slivers in the final product have been traced to reoxidation that originated from FeO in the ladle slag [2, 3,4].

Many countermeasures were adopted to lower these FeO and MnO contamination which
are shown below:

1.Minimized slag carryover from EAF to ladle during tapping
2.Increased aim turndown carbon
3.Avoiding reblowsfor minimizing the dissolved oxygen content in the steel
thereby reducing the amount of FeO in the furnace slag [2].
4.Ladle slag reduction treatment [2,4,7]

By minimizing slag carryover, together with adding a basic ladle slag and basic lining to lower the ladle slag to less than 1-2% FeO+MnO, can reduce total oxygen to 10 ppm for Low carbon aluminium killed steel. [5] Another way to lower the FeO+MnO content of the ladle slag is to add a slag conditioner (i.e. slag reduction or deoxidation treatment), which is a mixture of aluminum and burnt lime or limestone.

Casting Speed:

Casting speed plays a major role for generating slivers. Slivers may occur when there is entrapment of casting powder or mould powder. It mainly occurs when there is a variation in the casting speed thereby more chances of entrapment of these powders generating slivers.

The above figure 4 shows the places where there were chances of air entrapment from EAF tapping to Tundish region generating Al2O3 and FeO inclusions

Table 1 Origin of Al2O3 and FeO from different places Sources of Reoxidation :Ladle to Tundish
S.No                Parameters                  O2 pickup Al2O3 / FeO  generation
1                  Carryover slag            (average %P reversion is 0.002 and %FeO is 21) - 272 Kg of FeO
2             N2 pickup from ladle lifting to casting start     <4 ppm 1ppm of O2 2ppm of Al2O3
3           In Tundish moisture H2 pick up is 4 ppm of H2 32ppm of O2 64ppm of Al2O3
4         Ladle exchange time Silicon pick up is  >100ppm Si (Grade change from Si to Al killed steels      [11]100ppm of O2 200of Al2O3 (almost 400ppm N2 equivalent)

5  During tapping time through EBT (Tapping time is around 3-4 mints) 10ppm of O2     20ppm of Al2O3
6 Ladle transformation form EBT station to Ladle treatment position (Transformation time is 5 mints) 10ppm of O2 20ppm of Al2O3
7 During ladle open with lance at casting station [11] 10ppm of O2 20ppm of Al2O3,FeO
8 Shroud leakage [11]2ppm of O2 4ppm of Al2O3