Development of a mathematical model to estimate slag viscosity and
liquidus temperatures for four phase slag system in Blast Furnace
Vijaykumar, Anirudhadey, D.S.Vinoo and Dr.Marutiram.K
R&D department, JSW Steel Ltd. Vijayanagar Works, Toranagallu, Karnataka – 583275
E-mail: vijaykumar.verma@jsw.in; Ph: 08050738341
ABSTRACT
In the present work, viscosities and liquidus temperatures of high alumina blast furnace slags
were investigated. Viscosity of the quaternary synthetic slags system (CaO-SiO2-MgO-Al2O3)
was experimentally determined by the rotating cylinder method using Brookfield digital
viscometer model 2416. Experiments were conducted in the temperature range of 1450-1550 o C.
The effects of temperature, basicity, and MgO of slags on viscosity were studied. Synthetic slags
were prepared for various basicity compositions and the viscosity, liquidus temperatures were
experimentally determined.
Viscosity decreases and liquidus temperature increases with increase in basicity for high
alumina blast furnace slags. The CaO and MgO act as a network modifier and at the same time
they form compound such as calcium and magnesium di-silicates causing rise in liquidus
temperature.
Based on above experimental data, empirical relationships between slag composition and
liquidus temperatures were developed. Using the empirical relations, a mathematical model was
developed for the estimation of viscosity and liquidus temperature of the blast furnace hearth
slags. These models can be applied for determining the slag regime to achieve optimum slag
volumes. The model has been validated with plant data and performance.
Keyword: Viscosity, Liquidus temperature, Regression, Mathematical model, Slag regime,
INTRODUCTION
JSW Steel ltd. at vijayanagar 10 MT steel plant, fluxed Sinter is the main feed in BF-3 and BF-4.
Due to the iron ore crisis times, JSW steel was forced to operate a low grade ores with high
alumina uses in blast furnaces. High alumina percentage in iron bearing materials has resulted in
increased flux addition in sinter making. High alumina slag operations increase the slag viscosity
and liquidus temperature, casing difficult is for smooth blast furnace operation. Slag viscosity
and liquidus temperature are the extremely important characteristics of the slag in iron making
process. A viscous final slag can impair the hearth drainage, delay in casting, and effect the hotmetal quality. At JSW BF operations, the hearth slag, viscosity and liquidus are mentioned in the
range of (350-500 Centipoise) and liquidus temperature (1380-1440oC).
A number of researchers have worked on predicting the slag viscosity and liquidus temperature
in order to have been control on furnace operation, In 1957, Osborn et al.[1] systematically
investigated the liquidus temperature and crystalline phase of a CaO-MgO-SiO2-Al2O3 system
with 5 to 35 percent Al2O3. In 1965, Steyn and Watson [2] investigated the liquidus properties of
a high MgO blast-furnace slag. Snow [3] [U. S. Steel (USS) Research, 1964] presented 30 charts
of blast-furnace slag viscosity and melting temperature. Most of the slag phase diagrams were
subsequently collected in a reference book, Slag Atlas [4] (edited by VDEh) in 1995. During the
same period, various theoretical slag models were developed to describe the relationship between
the slag liquidus properties and slag chemical composition. For calculation of phase diagrams,
several thermodynamic models and databases have been established, such as CSIRO
FACT
[6].
For estimation of viscosity, the models of Riboud
[7]
and Urbain
[8]
[5]
and
are referenced the
most. This work presents the development of a mathematical model with specific configured to
Suit Blast furnace operations at JSW Steel.
METHODOLOGY
Methodology for the development of a mathematical model to estimate slag viscosity and
liquidus temperatures are given below in figure 1.
Synthetic slag
preparation
Milling synthetic slag
Briquette sample preparation
Liquidus temp. measurement
Chemical composition analysis
Viscosity measurement
Results compilation
Development of empirical
Relation
Development of model
Validation of model
Figure 1: Methodology for the development of a mathematical model.
Sample preparation
Samples were prepared from chemical ingredient like magnesia, silica, and alumina brick and
burnt lime which gives the nominal chemical compositions as resemble as blast furnace hearth
slag. The ingredients were ground and weighed to the desired compositions and mixed in a
mortar and then melted in a chamber furnace for 30 minute at 1500oC temperature for the
homogenization, these samples were crushed and ground into fine powders. These homogeneous
slag powders are ready to be use.
Viscosity measurements
In the present work, viscosity measurements were carried out by the rotating cylinder (bob)
method using rotating Viscometer (model Rheometer V). This equipment was supplied by Theta
Industries, USA. The furnace had a specially designed Molybdenum disilicide (MoSi 2) heating
element which was controlled by Eurotherm controller (Model 2416) by using Pt-30Rh
thermocouple. The slag Sample in the crucible was again melted in the vertical tubular furnace to
achieve the correct level of the slag inside the crucible. The furnace has a maximum continuous
temperature of 1600 oC, controlled by a PID controller unit. The programmable viscometer is
designed for measurement of viscosity at the given shear rates. The maximum accurate viscosity
measurement range is 2000 centipoises, and accuracy of viscosity measurement is -2.5 to +2.5%.
The temperature was measured by a Pt–30Rh thermocouple touching the crucible from the
outside, The crucible, filled with 80 g slag, was placed in the furnace and heated to 1550 oC with
heating rate of 10 oC/min, measuring the viscosity of slag with decreasing temperature up to a
possible lower limit when torque reached 75%.
RESULTS AND DISCUSSIONS
The viscosity of the selected quaternary slags system (CaO–MgO–Al2O3–SiO2) was measured in
a wide temperature range from 1450 to 1550°C.The synthetic slags contains MgO (6-10) wt%,
Al2O3 (18-22) wt% and mass ratio of the CaO/SiO2 were varied from (0.95 – 1.05) studied in
high temperature viscometer, and results are given below in graphical form.
Effect of temperature on viscosity
As shown in Figure- 2, viscosity decreased, as expected, with increasing temperature. It is also
observed that all of the slag is ‘short’ slags, whose viscosity increases abruptly with a small
decrease in temperature at or close to a certain temperature. In such slags, the temperature should
be closely monitored and should not be allowed to fall below a certain value in which case the
slag becomes highly viscous, thus hindering the reduction process as well as the separation of the
metal and the alloy from the slag. At higher temperature, since the slag temperature is still
considerably higher than the liquidus temperature, the increase in viscosity with a decrease in
temperature should not be significant. As some solid particles often precipitate from slags at and
below the liquidus temperature, the viscosities exhibit non-Newtonian behavior and show a sharp
1800
1600
1400
1200
1000
800
600
400
200
0
MgO = 6 wt %
MgO = 8 wt %
MgO = 10 wt %
1420
1430
1440
1450
1460
1470
1480
1490
1500
1510
1520
1530
1540
1550
1560
Viscosity (Centipoise)
viscosity increase near and below the liquidus temperature.
Temperature (oC)
Figure 2: The change in viscosity with respect temperature has been given for Alumina- 22wt%,
B2=1 and for three different (6, 8 and 10 wt%) of magnesium oxide. As well as we can see that
after a certain point viscosity changes steeper that is called liquidus temperature.
Effect of phase on viscosity and liquidus temperature
Viscosity and liquidus temperature of any slag depend on the phases present in the slag rather
than chemistry. These are the main phases observed in synthetic slag like Akermanite alumina
syn, Calcium aluminum silicate, Calcium magnesium silicate, Magnesium aluminum oxide and
Corundum, As well as from the XRD results one important observation is increasing the MgO wt
percentage magnesium aluminum oxide phase formation increases and corundum phase
decreases that results increase liquidus temperature and decrease in viscosity.
XRD Analysis
XRD analysis was done to get the mineralogical phases present in the slag samples. The
slag was first heated to 1500 °C for one hour followed by slow cooling. The purpose of this was
to convert the slag to crystalline form. Then XRD and microscopic analysis were done. From the
XRD results it is clear by increasing MgO wt% Magnesium aluminum oxide formation takes
place and the consequences liquidus temperature increases. As well as viscosity decreases
because MgO act as network modifier. XRD results are given below in figure 4.
Figure 4: XRD plots showing traces of mineralogical phases in case synthetic Slag A, B and C.
In all cases, Calcium magnesium silicate was found to be the major phase. However, some traces
of Akermanite aluminian syn, Magnesium aluminum oxide, Calcium magnesium silicate and
corundum syn were also present.
Table 1: Mean concentration of the main phases(Chemical formula) identified in the three
different synthetic slag samples like A(6 wt%), B(8 wt%) and C(10 wt%).
Sample A (6 Sample B ( 8 Sample C (10
wt % MgO) wt % MgO)
wt % MgO)
Phase
Chemical formula
Akermanite,
aluminian, syn
Ca2(Al0.46Mg0.54)((Al0.23Si0.77)2O7)
21.2
26.7
23.2
Ca(Al2Si2O8)
53.5
40.6
45.5
Calcium
aluminum
silicate
Magnesium
aluminum
oxide
Calcium
magnesium
silicate
Corundum,
syn
(Mg0.725Al0.272)(Al1.727M0.271)O4
10.1
Ca2Mg(Si2O7)
21.2
24.8
Al2O3
4
7.9
21.2
Structure of blast furnace slag
Crystal analysis of solid silica shows that silicon occupies the canter of a tetrahedron surrounded
by 4 oxygen atoms, one at each of the four corners. Each oxygen atom is bonded to two
silicon atoms and the network is continuous in three dimensions. These tetrahedral can
share only corners so that when every corner oxygen atom is shared, the substance formed will
have an overall stoichiometric formula of SiO2. A Si atom has 4 charges.
The group, (SiO4)4- which is regarded as individual tetrahedron with silicon at the centre and
oxygen at the four corners, can be assumed to exist as ion in the complex silicates.
Measurement of the energy of activation for electrical conductance and other results indicate that
the addition of CaO, MgO or other metal oxides to molten silica results in the breakdown of the
three dimensional silicon-oxygen network into silicate ions. The driving force for the breakdown
process is the attraction between silicon and oxygen. As the strong silicate networks break, the
viscosity of the melt decreases drastically as viscosity of a material depends not only on its
composition but also on its structure. In slag alumina also network compound and dissolving
mechanism same.
DEVELOPMENT OF MODEL
Empirical relation
This model has integrated the liquidus temperature data of the CaO-MgO-SiO2-Al2O3 system for
18 to 22 wt% Al2O3, 6 to 10 wt % MgO and 0.95 to 1.05 B2 from the experiment. The other
oxides, such as FeO, MnO, K2O, Na2O, and TiO2 have very little effect.
X = p (CaO) + q (SiO2) + r (Al2O3) + s (MgO) + t
Where X is liquidus temperature and p, q, r, s, and t are constant, they are varying with respect to
composition.
Similarly viscosity calculation of this model is based on the experiment data for the same
composition range above have mentioned as well as temperature range was from 1450 to
1550°C.
By using regression technique, total 188 data points were used for regression, which led to
establishing a quantitative relationship of how slag viscosity change with slag composition and
temperature and empirical relation are given below.
Y = a (CaO) + b (SiO2) + c (Al2O3) + d (MgO) + e
Where Y is viscosity and a, b, c, d, and e are constant, they are varying with respect to
temperature and composition.
Model interface
A PC version of blast furnace hearth slag mathematical model was developed firstly. The
interface of the PC model is shown in Figure 6. The user simply enters the slag chemistry, and
clicks the Run button. The slag viscosity and the liquidus temperature appear immediately. This
model has been applied for operation analysis. Several instances of using this model have helped
to determine that the furnace upsets were related to improper final slag or liquidus temperature.
It was therefore decided to modify it for on-line application.
Figure 6: PC version of the blast-furnace slag model for viscosity and liquidus temperature
calculation.
CONCLUSIONS
Empirical equations have been successfully derived by experimental data of the synthetic slag
viscosity and the liquidus temperature. Based on this analysis, an offline computer model was
developed for calculation of blast furnace slag viscosity and the liquidus temperature. The
results from the model matching very well with actual viscosity and liquidus temperature. It is
well known that blast furnace slag viscosity and liquidus temperature are extremely important for
achieving a smooth blast-furnace operation. This slag model provides offline guideline for plant
personnel to know the key properties of their blast-furnace slag.
REFERENCE
1. E. F. Osborn, R. C. Devries, K. H. Gee, and H. M. Kraner: Journal of Metals, January 1954,
pp. 33-45.
2. J. G. D. Steyn and M. D. Watson: Journal of the Iron and Steel Institute, May 1965, pp. 445453.
3. R. B. Snow: U. S. Steel Research Report, December 11, 1964.
4. Slag Atlas 2nd Ed., Verlag Stahleisen, Dusseldorf, 1995, pp. 157.
5. G. Turnbull, M. W. Wadsley: CISRO, Institute of Energy and Earth Resources, Port
Melbourne, Australia (1985).
6. W. T. Thompson, G. Eriksson, A. D. Pelton, and C. W. Bale: International. Symposium on
Computer Software in Chemical
and Extractive Metallurgy, Proc. Metall. Soc. Of CIM, Montreal, 1988, p. 87.
7. P. V. Rioud, Y. Roux, D. Lucas, and H. Gaye: Fachbe r. Huttenprax,. Metallweiterverarb.,
(19), 1981, pp. 859.
8. G. Urbain, F. Cambier, M. Deletter, and M. R. Anseau: Trans., British Ceramic Society (80),
1981, pp. 139.