A methodology to investigate the wear of blast furnace hearth carbon refractory lining
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Materials and Corrosion 2012, 63, No. 9999 DOI: 10.1002/maco.201106390 1
A methodology to investigate the wear of blast furnace
hearth carbon refractory lining
S. N. Silva, F. Vernilli, S. M. Justus, E. Longo, J. B. Baldo,
J. A. Varela and J. M. G. Lopes*
In this work it is presented a methodology applied by Companhia Siderúrgica
Nacional (CSN, Brazil) in order to assess its blast furnace 3 carbon refractory
hearth lining conditions. The aim of the investigation was the gathering of
critical data base for blast furnace refractories life cycle follow up and eventual
repair decision taking.
The sample drilling locations were chosen around the tap holes and hearth
bottom areas. The advancing drilling depths were based on local temperature
profiles. The isotherm limit of 500 8C was elected as a control parameter to
assess the critical carbon refractory condition. The guidelines for sampling and
testing as well as the results obtained through the physical and chemical
characterizations of the cored samples are presented. The condition of the
refractory lining is discussed under the light of the different known wear
mechanisms of blast furnace hearth carbon refractories.
1 Introduction micropore, dopping), were associated with the installation of
coolers in the blast furnace bottom. In large blast furnaces such as
It is a well-known fact that the refractory hearth lining is a critical CSNs 3, the wear is most intense in the bottom and its junctions
feature in the realm of the life expectancy of a blast furnace. In this with the side walls. The carbon refractories used to line the 3 blast
sense, invasive procedures were undertaken in order to assess the furnace hearth was of the supermicropore type.
carbon refractory hearth lining conditions of CSNs blast furnace 3, Refractories which are used in the blast furnace (BF) hearth
after producing 28 million tons of pig iron in 11 years of campaign. are subjected to the following chemical etching mechanisms:
The guidelines used for the sampling and the information put oxidation, alkali attack, CO disintegration, and erosion and
together by the investigation, conducted on cored specimens, were dissolution owing to hot metal and slag flow. In addition to the
important for a global understanding of carbon refractories hearth aforesaid chemical etching mechanisms, hearth refractories are
lining wear phenomenology, as well as for the setup refractories subjected to thermal stresses because of temperature fluctua-
selection guidelines and possible repair programs. tions, which may reach temperatures as high as 500 8C [1–3].
Historically every steel mill wants to increase its blast furnace
productivity. The first step commonly taken in this direction 1.1 Water vapor oxidation
consists in the increase of the processing temperature. As a
consequence the molten pig iron temperature increases, The oxidation of carbon bricks exposed to water vapor, depends
resulting immediately in an overall increase in the refractory basically on the vapor concentration, time of exposure and the
lining wear. This was particularly critical for the crucible lining temperature level. Oxidation tests [4, 5] conducted on several carbon
based on carbon refractories. In order to counterbalance the refractories (carbon, hot pressed carbon, and graphite) commonly
increased wear, the use of microstructure engineered improved used in the refractory lining of blast furnaces, indicated that the
graphite-based carbon refractories (higher thermal conductivity, oxidation by water vapor starts already at 450 8C. However, it is
generally considered that the potential risk for deterioration of blast
S. N. Silva furnace carbon refractories by water vapor is only marginal at
Companhia Siderúrgica Nacional (CSN), Rio de Janeiro (Brazil) temperatures below 500 8C. The heat flux through the refractory
lining is also an important parameter to be taken into account. In the
F. Vernilli, J. M. G. Lopes service face of the carbon refractory lining it is created an adhered
Engineering School of Lorena, EEL/USP, São Paulo (Brazil) layer composed mostly of coke and slag. This layer displays a much
E-mail: fernando.vernilli@usp.br
smaller thermal conductivity than the carbon refractory. If the
S. M. Justus, E. Longo, J. B. Baldo, J. A. Varela adhered layer thermal conductivity and thickness provide conditions
Multidisciplinary Center for Development of Ceramic Materials, to keep the hot face temperature below 1150 8C (solidification
CMDMC/UNESP, São Paulo (Brazil) temperature of molten pig iron), it will help to protect the lining.
www.matcorr.com wileyonlinelibrary.com ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim2 Silva et al. Materials and Corrosion 2012, 63, No. 9999
Finally, one very important operational parameter which In general, carbon-based refractories have a high thermal
affects blast furnace life cycle is the leakage of the cooling water. conductivity and low expansion coefficient [7]. As a result, the
Special care must be devoted to the copper plates being sure that tensile strength becomes high and thus, less susceptible to failure
the water is sealed in the region located below the tuyeres. If by by thermal shock.
any means leakage occurs, water can get into the crucible wall Table 1 shows the highest increase in temperature allowed
triggering oxidation and even pushing the hot face adhered layer for different materials. As we can see, carbon-based refractories
off. As a result sudden and huge wall temperature increases may are the unique material that could work with intense increases in
be experienced, which will lead to accelerated wear of the carbon temperature [9].
refractory lining [6].
1.4 The effect of infiltrated pig iron
1.2 Alkali and zinc vapor attack
When the coating layer at the carbon refractory surface is broken
The attack by alkali and zinc vapor occurs by permeation of the or washed away, then the molten pig iron can penetrate inside the
gaseous species through the refractory open porosity. The bulk of the carbon block. Normally, it is considered that the
penetration depth depends on the wall temperature profile, penetration by pig iron, depends on several aspects [10]:
product pore structure, and overall permeability. The alkali attack
is generally limited by the 800 8C isotherm and increases with the (i) Surface tension of the molten pig iron.
temperature. The alkali vapors are able to intercalate between the (ii) The molten bath column pressure over the carbon lining.
carbon lamellae forming carbon potassium compounds such (iii) The contact angle between the molten pig iron and the
as C8K, C24K, and C60K. They also react with the aluminum carbon refractory.
silicate rich ashes in the carbon blocks forming low melting point (iv) The volume fraction of pores in the range from 1 to
and expansive phases such as Kaliophilite (K2O Al2O3 2SiO2) 5 mm.
and Leucite (K2O Al2O3 4SiO2) [7]. The result is the embrittle-
ment and spalling of the carbon refractory. The extension of alkali The first three parameters are a consequence of operational
attack also depends on the kind of the carbon source in the conditions, the fourth one has to do with the refractory itself.
refractory. The more the carbon is in graphitic form, the less the Normally, the surface tension of the molten pig iron is about
chemical reaction with the alkali [6, 7]. 600 MN/m and its contact angle with carbon refractories is of the
order of 1408. It is assumed that under these conditions, a molten
1.3 The effect of thermal gradients pig iron column height of 3 m is not able to penetrate into pores
smaller than 5 mm of apparent diameter.
Sudden temperature changes may result in the nucleation and Several phenomena can take place when pig iron manage
propagation of cracks in any refractory lining. The blast furnace to penetrate into the carbon block microstructure. The most
crucible is a dynamic environment specially around the tap holes, important of them are:
where thermal shock invariably occurs during the many tapping
cycles. (i) Structural spalling (brittle zone) caused by the difference
Following Hasselman’s [8] unified theory of thermal shock in thermal expansion coefficient between the pig iron
using thermal elastic approach, the critical sudden temperature penetrated zone and the sound refractory part.
change able to promote a critical crack nucleation, is given by (ii) The undesirable presence of iron. It can actuate as a catalyst
Equation (1) for carbon monoxide disintegration of the refractory
microstructure, at the isotherms below 600 8C.
s f ð1 2mÞ (iii) Dissolution of the carbon refractory in the undersaturated
DTC ¼ (1) pig iron.
cEa
The key factors governing the phenomenology of pig iron
where sf is the tensile strength of the material, m is its Poisson’s penetration are the cooling efficiency and the thermal con-
coefficient, E the material elastic modulus, a the reversible linear ductivity of the lining. These will control directly the thickness of
thermal expansion coefficient, and c is a function of the Biot’s
modulus, Equation (2). Table 1. Highest increase in temperature allowed for different
materials [9]
ah Material Maximum increase allowed
b¼ (2)
K in temperature (8C/min)
Alumina 44% 4
where K is the thermal conductivity, h the film heat transfer Alumina 85% 5
coefficient, and a is a geometric factor given by the specimen Cast iron 50
geometry and dimensions. Silicon carbide 50
From the above equation it is clear that the smaller the Semi graphite 250
Graphite 500
thermal expansion coefficient and the bigger the ratio (s f/E), the
Blast furnace real measure 150
more the material resists cracking nucleation.
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.comMaterials and Corrosion 2012, 63, No. 9999 Investigation of blast furnace hearth carbon refractory lining 3
the adhered layer, the carbon lining hot face temperature and the other hand a too difficult progress of the drill machine is an
freezing conditions of the molten metal. Low thermal con- indication of pig iron penetration. These facts must be properly
ductivity carbon refractories lead to higher hot face temperature registered and further investigations of the problem must be
and thinner adhered layer. In this sense, in order to keep the hot considered.
face temperature below 500 8C for a normal thermal load Special care (covers) must be devoted to avoid water
(10 000 W/m2) operational mode, it is a must to use carbon infiltration of the drilled sampling holes during drilling.
refractory linings whose thermal conductivity is not less than The sampling locations in the surroundings of tap holes 1, 3
25 W/mK. On the other hand, in order to keep the hot face and 2, 4, and their respective latitudes are indicated on Figs. 1
temperature of the lining below 1000 8C, when working under and 2 and the typical drilled samples are shown in Fig. 3.
high thermal loads (50 000 W/m2), the thermal conductivity of After the drilling operation was ended, starting from the cold
the carbon hearth refractory should be over 75 W/mK [11]. face of the carbon refractory lining inside the drilled holes,
thermocouples were installed at three different depths (at the
2 Step by step procedure shell surface, 50 and 250 mm). As a safety measure, carbon rods
were also inserted into the drilling holes to fix the thermocouples.
The tracking of a blast furnace hearth carbon refractory wear The extremity of the carbon rod was covered with a refractory
by an invasive procedure must be done very carefully. The carbon mortar in order to guarantee a perfect connection between
methodology used in this work was based on strict temperature the thermocouple and the carbon lining.
monitoring during the sequential drilling of the steel shell, Aiming to get comparative results, reference specimens were
the ramming mass, and the carbon lining itself. The drilling drilled from unused carbon block of the same quality and lot used
stopped at a specific depth or each time the measured in the original lining. These were submitted to the same testing
temperature reached 500 8C. By means of the appropriate program as the ones taken from the blast furnace.
sensors, carbon monoxide and other hazardous gases monitoring
was made continuously during the drilling operation. The drilling 2.1 Specimen preparation
direction was always made horizontally.
The drilled procedure was as follows: In order to get a more representative portion from the drill bit,
the investigation specimens were extracted from the core of the
(i)Drilling of the steel shell. drilled samples as shown in Fig. 4. For chemical analysis the
(ii)Measurement of the ramming mass cold face temperature. specimens were pulverized in an agate mortar and sieved to pass
(iii)Drilling of the ramming mass. a 100 mesh sieve.
(iv) Measurement of the ramming mass thickness and its hot In order to have an initial testimony, the drilled samples were
face temperature, which happens to be the same as the visually inspected for signs of alkali vapor spalling, pig iron
carbon lining cold face temperature. penetration and the presence of ashes. After the visual inspection,
(v) Start of carbon lining drilling at increments of 100 mm. all of the samples were photographed.
Always measuring the temperature.
(vi) Stop drilling when the measured temperature of the carbon 3 Results and discussion
lining reached 500 8C.
3.1 Visual inspection
The drilling operation of the carbon lining must be done very
slowly and any sudden easiness on advancing the drill machine is Visually it was not found any critical damage in the drilled
an indication of a disintegrated or cracked lining zone. On the samples except the one in the neighborhood of tap hole 1 at
Figure 1. Sampling locations and respective elevations in the surroundings of tap holes 1 and 3
www.matcorr.com ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim4 Silva et al. Materials and Corrosion 2012, 63, No. 9999
Figure 2. Sampling locations and respective elevations in the surroundings of tap holes 2 and 4
latitude of þ6900 mm. This sample presented cracks in its It is important to know that in this latitude (þ5700 mm) the
extremity indicating a possible brittle zone. wear is more intense mainly due to the erosion caused by liquid
flux on the peripheral region where a typical wear profile comes
3.2 Alkali attack up. Figure 6 shows the shape of this wear called deadman packing
(hot metal carbon saturation). In addition the temperature profile
Chemical analysis done on all the specimens indicated that the shown in Fig. 6, due to erosive wear explained above, allows
potassium contents were always higher than the sodium ones. alkalis infiltration in greater depths [7].
The highest potassium concentrations in the bottom area were
found in the depths ranging from 900 to 1400 mm from the steel 3.3 Water oxidation
shell over an isotherm of approximately 800 8C. This fact indicates
that the alkali attack occurs via vapor phase condensation. In In order to evaluate the eventual oxidation of the carbon blocks
general the determined potassium contents were under 0.3 wt%. promoted by water vapor, the fixed carbon content was taken as an
However, samples taken in the vicinity of tap hole 1 at latitude of
þ5700 mm reached 0.969 wt% as shown in Fig. 5.
Figure 4. Location of the specimen taken from the drilled samples to
be used in the investigation
Figure 5. Alkali concentrations (sodium, potassium) as a function of
the drilling depth on the carbon block. Sample extracted in the vicinity
Figure 3. Typical drilled samples taken from the blast furnace hearth of tap hole 1 and latitude þ5700 mm
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.comMaterials and Corrosion 2012, 63, No. 9999 Investigation of blast furnace hearth carbon refractory lining 5
Table 3. Characterization results of samples taken at latitude
þ6900 mm in the surroundings of tap hole 1
Properties Drilling depths (mm)
325 625 925 1225 1375
Fixed carbon (%) 92.36 92.60 89.82 91.59 92.84
Volatile matter (%) 0.94 0.94 0.99 0.97 0.97
Ashes (%) 6.47 6.26 8.65 6.89 5.69
Moisture (%) 0.22 0.19 0.53 0.55 0.49
CaO (%) 0.116 0.092 0.210 0.112 0.104
ZnO (%) 0.003 0.002 0.002 0.003 0.003
Fe2O3 (%) 0.209 0.170 0.247 0.172 0.850
K2O (%) 0.097 0.317 0.229 0.343 0.093
Na2O (%) 0.070 0.064 0.084 0.096 0.066
Apparent porosity (%) 11.4 11.4 10.9 10.5 –
Average pore diameter (mm) 9.4 2.7 7.7 4.3 –
Apparent density (g/cm3) 1.55 1.57 1.55 1.52 –
Structural density (g/cm3) 1.75 1.78 1.75 1.69 –
True density (g/cm3) – – – – 1.95
Figure 6. Typical wear profile for BF hearth. 1: lost layer (eroded and conclusion that oxidation by water may have occurred and the
dissolved); 2: protective layer (scab); 3: hot metal penetrated layer; microstructure became more permeable. However, the lower
4: brittle zone; 5: slightly changed layer; 6: unchanged layer [7] fixed carbon contents are associated more strongly to the higher
ashes contents. This fact indicates that the material inherent
indicative parameter of the oxidation extension. In all the samples chemical and mineralogical composition and not the water
investigated (except two in the surroundings of tap hole 1), the penetration, was the main cause of degradation in the
determined fixed carbon contents were above 92 wt%. This is surroundings of tap hole 1. The higher CaO contents found in
quite close to the nominal typical value for unused carbon blocks. the þ5700 mm latitude might indicate that this region has a less
This fact indicates that oxidation was not a wear issue in the thick residual lining at that time of the campaign.
studied case. The exceptions again were related to the latitudes
þ5700 and þ6900 mm in the region of the tap hole 1. Relevant
data from this problematic tap hole 1 surroundings at the two 3.4 Slag attack
latitudes (þ5700 and þ6900 mm) are presented in Tables 2 and 3.
We may notice that some samples displayed fixed carbon Except for the case treated above, the great majority of the
levels below the minimum of 92 wt% and moisture contents investigated samples presented low CaO contents. In average,
above 1 wt%, which is the tolerable limit. A global analysis shows these were quite close to the content of the unused carbon block
that lower fixed carbon contents are associated with higher (0.096 wt%). It may be concluded that globally there was very little
moisture and alkali contents. This finding may lead to the slag diffusion through the carbon blocks microstructure.
Table 2. Characterization results of samples taken at latitude þ5700 mm in the surroundings of tap hole 1
Properties Drilling depths (mm)
274 574 874 1174 1474 1774 1924
Fixed carbon (%) 92.87 91.80 88.31 87.12 88.98 91.53 89.69
Volatile matter (%) 0.79 1.18 1.56 1.47 1.45 0.97 1.10
Ashes (%) 5.96 6.10 8.72 9.48 8.20 7.01 8.19
Moisture (%) 0.36 0.91 1.40 1.92 1.36 0.48 1.01
CaO (%) 0.087 0.129 0.170 0.168 0.098 0.077 0.093
ZnO (%) 0.007 0.011 0.006 0.005 0.004 0.004 0.004
Fe2O3 (%) 0.295 0.257 0.315 0.362 0.268 0.363 0.247
K2O (%) 0.128 0.242 0.968 0.910 0.667 0.094 0.587
Na2O (%) 0.071 0.075 0.170 0.161 0.130 0.064 0.097
Apparent porosity (%) 7.9 9.1 10.2 10.8 9.9 10.5 9.9
Average pore diameter (mm) 6.5 1.8 3.9 2.7 4.7 2.4 1.2
Apparent density (g/cm3) 1.60 1.57 1.06 1.59 1.54 1.54 1.56
Structural density (g/cm3) 1.72 1.73 1.18 1.78 1.71 1.72 1.73
True density (g/cm3) – – – – – – 2.08
www.matcorr.com ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim6 Silva et al. Materials and Corrosion 2012, 63, No. 9999
3.5 Zinc penetration
Considering the measured zinc contents in all samples were very
low it may be concluded that zinc was not a wear factor for the
carbon blocks during the campaign.
3.6 Pig iron penetration
The iron oxide content found in the unused carbon block was
relatively high (0.24 wt%). In the great majority of the investigated
samples, the iron oxide content was similar to the unused sample.
Considering that the registered thermal history of the hearth
during 11 years of campaign did not show any abnormal
temperature fluctuation, we may conclude that pig iron did not
penetrated the carbon blocks during the campaign. However, an
exception was found in the surroundings of tap hole 1 at a depth
Figure 8. Back scattered electron micrograph (A and C) and X-ray
of 1312 mm in the þ6900 latitude. From this depth on, it was mapping of iron (B and D) in the specimens 18 and 1, respectively
found a typical brittle zone characterized by spalled parts and
cracks distributed in a parallel way. One specimen taken at the
depth of 1375 mm presented an iron oxide concentration of carbon block under study was of the supermicropore quality. This
0.85 wt% which is three times greater than the one of the unused implies that the great majority of its pores are virtually
carbon block. nonpermeable to the molten metal. Most certainly, before the
In order to get a better understanding of this abnormal wear pig iron could penetrate into the carbon block, some initial
zone, an SEM investigation was set up. The center line of drilled damage in the carbon block microstructure must have happened.
samples at depths between 1312 and 1492 (latitude þ6900 mm), As discussed previously, alkali vapors have a penetration potential
were cut into 18 cubic specimens of 1 cm3 as schematically in the depths corresponding to isotherms greater than 800 8C. We
shown in Fig. 7. These specimens were prepared for electron also know that the pig iron solidification isotherm occurs about
microscopy. 1100 8C. We may conclude that in this case, the carbon block wear
The X-ray mapping of specimen 18 (spalled off region), mechanism starts by the action of alkali vapor penetration
displayed in Fig. 8, presented a region like a crevice rich in iron. through the connected pores in the carbon block microstructure.
On the other hand specimen number 1 (nonspalled region), Next the reaction with the carbon ashes occurred producing
displayed an iron distribution typical of the unused block. In expansive phases. This resulted in the cracking and the increase
Fig. 9 it is shown how the iron counts increase with respect to the in pore size and pore volume fraction, degrading the mechanical
depth for the specimens in the degraded zone. In Figs. 10 and 11 properties of the original microstructure. The degraded micro-
the image analysis and microprobe data, confirm that the spalled structure was not able anymore to halt molten pig iron
zone close to the 1492 mm depth, was due to the brittleness penetration. This hypothesized wear mechanism was confirmed
caused by pig iron infiltration. by the ray analysis shown in Fig. 12. It was detected the presence
However, a question still remained of how the pig iron of the expansive alkali alumino silicate phases Kaliophilite and
managed to penetrate in a material with such fine pores? The Leucite.
Figure 7. Schematic of the cut 18 specimens between the depths of
1312 and 1492 mm cut for SEM investigation Figure 9. Results of iron X-ray counting analysis as a function of depth
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.matcorr.comMaterials and Corrosion 2012, 63, No. 9999 Investigation of blast furnace hearth carbon refractory lining 7
Figure 12. X-ray diffraction of the carbon block alkali damaged zone in
specimen 18
that time of the campaign. The decisions taken based upon the
information gathered by the use of the methodology were enough
to set up a repair program for the tap holes surroundings which
prolonged the blast furnace 3 campaign for additional 5 years.
Concerning susceptible to failure by thermal shock, carbon-
based refractories are the most appropriated material for this job. It
can be explained due to its high thermal conductivity and low
expansion coefficient, nevertheless erosive corrosion allows infiltra-
tion of alkalis in greater depths than usual accelerating the wear.
Figure 10. Image analysis of specimens 14 (A), 15 (B), 16 (C), 17 (D), and The adoption of 500 8C isothermal standard safety limit for
18 (E). The bright areas indicate iron rich zones
blast furnace heart refractory carbon blocks was a valuable check
point parameter for the survey of the carbon refractory lining.
Acknowledgements: The authors thank FAPESP, CNPQ, and
FINEP for the financial support of this investigation.
5 References
[1] F. Vernilli, S. M. Justus, A. Mazine, J. B. Baldo, E. Longo, J. A.
Varela, S. N. Silva, Mater. Corros. 2005, 56, 475.
[2] F. Vernilli, S. M. Justus, S. N. Silva, J. B. Baldo, E. Longo, J. A.
Varela, ISIJ Int. 2005, 45, 1871.
[3] C. Xilai, L. Yawei, L. Yuanbing, J. Shengli, Z. Lei, G. Shan,
Metall. Mater. Trans. A 2009, 40, 1675.
[4] T. Talaat, Internal Report, Hoogovens Technical Services, 1994.
[5] K. Hattori, E. Takemura, H. Shibata, Nippon Steel Tech. Rep.
1975, 7, 94.
[6] S. M. Justus, R. M. Andrade, S. N. Silva, O. R. Marques, J. M.
Figure 11. Results of iron image analysis as a function of depth Rivas, S. Cava, L. E. B. Soledade, I. M. G. Santos, J. B. Baldo,
C. A. Paskocimas, E. R. Leite, R. J. A. Varela, E. Longo, Bol.
Soc. Esp. Cerám. Vidrio 2002, 41, 233.
4 Conclusions [7] S. N. Silva, F. Vernilli, S. M. Justus, O. R. Marques, A.
Mazine, J. B. Baldo, E. Longo, J. A. Varela, Ironmaking
The surveying invasive methodology employed to assess the Steelmaking 2005, 32, 459.
conditions of the 3 blast furnace hearth carbon refractory lining, [8] D. P. H. Hasselman, J. Am. Ceram. Soc. 1969, 52, 600.
showed to be appropriate and provided important information [9] G. J. Tijhuis, N. G. J. Bleijendaal, Steel Times Int. 1995, 69, 26.
concerning the conditions of the hearth lining after 11 years [10] M. Nitta, Nippon Steel Tech. Rep. 2006, 94, 122.
operating and 28 million tons of pig iron produced. [11] M. Spreij, M. C. Franken, J. Trouw, G. J. Tijhuis,
The elected testing checklist was a valuable tool considering In Proceedings of the UNITECR’95, Kyoto, Japan
that the results indicated with fidelity the real lining condition at November 19–22 1995, pp. 165–175.
(Received: October 6, 2011) W6390
(Accepted: March 29, 2012)
www.matcorr.com ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimYou can also read
