PROTECTION MECHANISMS FOR BLAST FURNACE CRUCIBLE USING TITANIUM OXIDES

Abstract

In modern steelmaking the duration of a working campaign for a blast furnace is related to the life of the crucible. Adding titanium oxide has been a frequent practice in the operation routines for modern blast furnaces, seeking the protection of the crucible walls, independently from its physical or chemical characteristics. These practices, as conventional operation of iron and steelmaking installations, present both advantages as well as undesirable consequences. The work proposes the incorporation of rutile (TiO 2) or illmenite (FeO·TiO 2) in the refractory matrix of the linings, as a practice that results in a protection of the crucible without altering, under any circumstance, the regular operation of the installation.

Full text

Association of Metallurgical Engineers of Serbia
AMES Scientific paper
UDC: 669.162.212
PROTECTION MECHANISMS FOR BLAST FURNACE CRUCIBLE
USING TITANIUM OXIDES
Javier Mochón1*, Maria Jose Quintana2, Iñigo Ruiz-Bustinza3, Roberto
González Ojeda2, Erika Marinas Garcia3, Miguel Ángel Barbés Fernández3,
Luis Felipe Verdeja González3
1Departamento de Metalurgia Primaria del CENIM-CSIC. Avda. Gregorio del
Amo 8, Madrid, 28040, España
2Escuela de Ingeniería, Universidad Panamericana. Augusto Rodin 498,
México, D.F. 03920, México
3Grupo Sid-Met-Mat de la Universidad de Oviedo, Unidad Asociada al
Departamento de Metalurgia Primaria del CENIM. Escuela de Minas,
Independencia 13, Oviedo, 33004
Received 27.10.2011
Accepted 16.01.2012
Abstract
In modern steelmaking the duration of a working campaign for a blast furnace is
related to the life of the crucible. Adding titanium oxide has been a frequent practice in the
operation routines for modern blast furnaces, seeking the protection of the crucible walls,
independently from its physical or chemical characteristics. These practices, as conventional
operation of iron and steelmaking installations, present both advantages as well as
undesirable consequences.
The work proposes the incorporation of rutile (TiO2) or illmenite (FeO·TiO2) in the
refractory matrix of the linings, as a practice that results in a protection of the crucible
without altering, under any circumstance, the regular operation of the installation.
Key words: blast furnace, campaign extension, crucible, titanium dioxide additions.
Introduction
The existence of a blast furnace and its auxiliary installations (sinter, coke and
pelletizer plants), are still indispensable in the 21st century steelmaking industry.
Around 65% of the world’s steel production is made with it; and in an economic
consideration, 60% of the production costs for steelmaking as a whole are related to the
functioning of its installations in a correct fashion [1,2].
* Corresponding author: Javier Mochón, jmochon@cenim.csic.es

196 Metall. Mater. Eng. Vol 18 (3) 2012 p. 195-201
Therefore, during the last decades, all the problems related to the extension of the
working campaign of a furnace along with assuring high production rates of pig-iron,
have been widely discussed in the specialized bibliography. Even though while
prolonging the working campaign of a blast furnace, a large number of variables must
be taken into account, the work focuses only on those operation practices related to the
addition of titanium dioxide, with the purpose of repairing crucible zones affected by
considerable wear (specially on below the taphole or at the corners of the hearth):
formation of “elephant foot” or “mushroom type” profiles as shown in Figure 1 [3,4,5].
One phenomenon that must be considered when studying the protection of
furnaces and pyrometallurgic reactors is the formation of scabs/accretions at the
interface of the lining with the corrosive fluid. Fredman [6] analyzed the benefits and
disadvantages that scabs in the stack/bosh zones of the blast furnace may cause in a
general (not specific) way, though some references are indicated to study the
consequences of accretions forming in the crucible.
Since the 1950s, the addition of titanium oxides in the material fed to the blast
furnace has been considered as an alternative to the protection of the crucible, as the
precipitation of titanium carbide, TiC, is expected. However this operation practice was
uncommon until the beginning of this century [7]. These practices include the addition
of titanium minerals (illmenite or rutile) inside the ferric charge fed to the furnace inlet,
in a continuous or discontinuous way (figure 1). Likewise, taking advantage of the
generalized use of injection technologies of carbon through the tuyeres, the selective
injection of titanium minerals is done, at zones surrounding the damaged lining, as
shown in figure 1 [8,9].
Barrel/
Figure 1. Blast furnace: thermal control, titanium dioxide additions and crucible wear
profiles

Mochón et al.-Protection mechanisms for blast furnace crucible using titanium oxides 197
Protection mechanism by adding titanium compounds
STAGE-I
STA GE-II
ST AGE-II I
Addition of TiO2 in the Slag
SLAG
Disolution of TiO2 in the Slag
Incorp ora tion o f TiO2 t o the
p
ig-iron as metallic titanium
CARBON
REFRACTORY
Precipitation of Titanium Carbide
Zone with the lowest heat
transport resistence
1−i
T2−i
T
Figure 2. Protective mechanism for a carbon refractory on zones with wear through the
precipitation of titanium carbide, TiC
Figure 2 presents the protection mechanism of a zone with wear in the
crucible, through the precipitation of titanium carbide, TiC, or complex titanium
carbonitrides, TiCxNy; it may be described by the following stages [10,11]:
Stage-I: Addition/dissolution of titanium oxides from the mineral charge towards the
slag.
Stage-II: Reduction of titanium inside the slag and its incorporation to the pig-iron:
TiO2(dis; slag)+2C(dis; pig-iron)⇔2CO(g)+Ti(dis; pig-iron) (1)
Stage-III: Precipitation/protection of TiC at zones with high wear in the crucible:
Ti (dis; pig-iron)+C(dis; pig-iron)⇔TiC(s) (2)
From the thermodynamic point of view, reaction (2) is exothermal and consequently
the equilibrium constant diminishes as temperature rises. If free energy, GΔ,
associated to reaction (2) is analyzed, its variation as temperature rises becomes less
negative (or more positive): precipitation of titanium is promoted when temperature
lowers [12]. This circumstance will explain why, at equal conditions, titanium
carbide will thermodynamically tend to precipitate at zones of the lining most
affected by wear:
21 −− >ii TT in Figure 2 (3)
where 1−i
T is the nodal temperature at the lining wall and 2−i
T is the temperature at
the maximum wear zone.

198 Metall. Mater. Eng. Vol 18 (3) 2012 p. 195-201
Experimental tests confirm that the reading of the thermocouples located at the cold
zones of the lining and affected by wear are higher than the ones at zones which are
not eroded. According Fourier’s First Law, the resistance to heat flux, q
J( W·m-2),
would be lower at zones with wear due to the reduction in the ratio:
⎟
⎠
⎞
⎜
⎝
⎛Δ
λ
x
(4)
where xΔ is the thickness (m) and
λ
is the thermal conductivity of the lining
(W⋅m-1⋅°C-1). Nevertheless, taking the Le Chatelier Principle into account (principle
of action and reaction from a chemical point of view), the system will have a
tendency to react by opposing itself to the increase in heat flux caused by the
diminishing in the thermal conductivity resistance at the damaged zones of the lining
[12]. Therefore, a heat flux reduction will take place through a diminishing in the
thermal gradient,
T
Δ:
()
coldi TTT −=Δ (5)
Consequently, a reduction in
T
Δ will be produced, either by a reduction of the
nodal temperature at the interface between refractory and pig-iron, 2−i
T of Figure 3,
or by an increase in temperature at the thermocouples of the cold zone of the lining
(500
cold
TC≤°
).
STAGE -I
SLAG
OXIDE-BASED
(CERAMIC CUP)/
CARBON-BASED
REFRACTORY
Reduction-Precipitation process of
Titanium Carbide, TiC, at the refractory
and pig-iron interface
Zone with less resistance
to heat t ransport
1
−
i
T
2
−
i
T
Zone/Sheet with
high concentration
of TiO2
Zone/Sheet with
high concentration
of TiO2
PIG-IRON
STAGE -I
SLAG
OXIDE-BASED
(CERAMIC CUP)/
CARBON-BASED
REFRACTORY
Reduction-Precipitation process of
Titanium Carbide, TiC, at the refractory
and pig-iron interface
Zone with less resistance
to heat t ransport
1
−
i
T1
−
i
T
2
−
i
T2
−
i
T
Zone/Sheet with
high concentration
of TiO2
Zone/Sheet with
high concentration
of TiO2
PIG-IRON
Figure 3. Protective mechanism for any kind of refractory quality at zones with wear
through the precipitation of titanium carbide, TiC, in laminated ceramic compounds

Mochón et al.-Protection mechanisms for blast furnace crucible using titanium oxides 199
New protection system proposal
Addition of titanium mineral to the blast furnace charge with the purpose of
protecting the zones of the crucible affected by considerable wear (Figure 2) has the
following disadvantages:
1. The performance of the precipitation of titanium is low, which is why a
discontinuous treatment with 20∼30 kg of TiO2 per ton of pig-iron is preferred,
compared to continuous addition campaigns of 3∼5 kg of TiO2 per ton. Both from
the kinetic as well as the thermodynamical point of view, it is advisable for the
amount of titanium in the pig-iron to be at least 0.10% [7,11].
2. Due to the low performance of the titanium additions, in recent years it
has been preferred to make selective treatments to those zones most affected by
wear, by injecting through the tuyeres most adequate for each case, the titanium
compounds [7,11].
3. Any of the treatments previously mentioned, requires an specific coke
consumption that represents the most important variable both in the operation as in
the cost of the blast furnace [7,11].
4. The presence of amounts close to 0.10% of titanium in the pig-iron,
results in a content of TiO2 in the slag between 1.0 and 2.0%. The properties of the
slag, such as density and viscosity, are substantially modified and increased
(specially viscosity), changing the patterns of the furnace cast: number and duration
of casts per day, decreasing the productivity of the furnace as viscosity becomes
higher [4,5,7,11,12].
5. In all the revised bibliography, the only substrate (heterogeneous
nucleation) for the precipitation of TiC or TiNC, is the carbon-based material. There
isn’t any information about the possible formation of these compounds on substrates
different than carbon [13,14].
Just as previously mentioned and without considering the inconvenience of the
presence of titanium in operations and processes subsequent to the blast furnace, this
work proposes an alternative route to reach the objectives sought: prolong the
operation life of the furnaces through the precipitation of TiC or TiNC at damaged
zones of the crucible, avoiding the disadvantages produced by existing conventional
practices.
Taking into account solutions currently applied by scientists and engineers in the
aeronautical field, and applying them to the design and construction of blast furnaces
[13,14], the most recent structural designs used for fuselage, wings or rudder: metal-
ceramic-polymer laminated compounds, new ceramic laminated compounds for blast
furnaces are being designed just as shown in figure 3. For these new crucible designs, it
will be possible to find zones of the lining (thin sheets): materials with high percentage
of TiO2 that react with the metallic melt or the slag when the erosion of the crucible
facilitates the contact between them.
The use of linings with laminated ceramic compounds may avoid most of the
disadvantages of current conventional treatments, as in this type of lining the
formation mechanism of titanium carbide will only be activated when the zones with
high TiO2 content make contact with the pig-iron. In this case, the protective
reactions will be:
TiO2(lining)+3C(dis; pig-iron)⇔2CO(g)+TiC(s) (6)

200 Metall. Mater. Eng. Vol 18 (3) 2012 p. 195-201
3(FeO·TiO2)(lining)+13C(dis; pig-iron)⇔9CO(g)+Fe3C+3TiC (7)
Supposing that the ratio:
()
()
1.0≈
CaCOP
(8)
as both the carbon activity in the pig-iron,
()
Ca , and the partial pressure of the carbon
monoxide,
()
COP , in the crucible of the blast furnace, are very close to its standard
activity/unit pressure states. The free energy associated to reaction (6) starts showing
negative values when T > 1200 ºC, while for reaction (7) the same happens, but for T
> 1100 ºC (1,10).
Finally, the presence in the design of the crucible of laminated ceramic compounds
with enriched TiO2 zones with thickness between 20∼40 mm (Figure 3) may be
compatible with the expansion zones needed (2D expansion in x and y) in the
crucible at the start of operation [11,13].
Conclusions
Though it’s an operational practice to use the protection mechanisms of carbide
and carbonitride precipitation of titanium using titanium minerals as addition to the
blast furnace charge, it results in low performance and affects operative variables and
costs of the steelmaking process.
An alternative to these problems is to design the crucibles with laminated
ceramic materials containing high titanium oxide sheets, in order to produce
precipitation at zones where erosion facilitates the contact of pig-iron with high titanium
content sheets.
Acknowledgements
The authors wish to acknowledge the Ministerio de Educación y Ciencia-España
(MEC): MAT2003-00502, the Ministerio de Asuntos Exteriores y Cooperación
(MAEC): MAEC-AECID-B/1629/04; B/2884/05; B/5814/06, B/7648/07, the FICYT de
Asturias and the CSIC-Madrid for facilitating the scientific and technological
cooperation between CENIM and Universidad de Oviedo.
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List of symbols, abbreviations and acronyms
GΔ free energy
1−i
T nodal temperature at the lining wall
2−i
T nodal temperature at the maximum wear zone
q
J heat flux
xΔ wall thickness
λ
thermal conductivity of the lining
T
Δ thermal gradient
i
T nodal temperature
cold
T temperature at the cold zone of the lining
()
COP partial pressure of carbon monoxide
()
Ca carbon activity in the pig-iron