X. WANG et al.: THERMODYNAMIC PROPERTIES OF CaO-SiO2-MgO-Al2O3-B2O3 SLAG SYSTEMS AT 1773 K
639–643
THERMODYNAMIC PROPERTIES OF CaO-SiO
2
-MgO-Al
2
O
3
-B
2
O
3
SLAG SYSTEMS AT 1773 K
TERMODINAMSKE LASTNOSTI @LINDERNEGA SISTEMA
CaO-SiO
2
-MgO-Al
2
O
3
-B
2
O
3
PRI TEMPERATURI 1773K
Xianmeng Wang, Yongchun Guo
*
, Mengyao Li, Yanqiang Bu
Xi’an Aeronautical Polytechnic Institute, Xi’an 710089, China
Prejem rokopisa – received: 2023-05-30; sprejem za objavo – accepted for publication: 2023-10-20
doi:10.17222/mit.2023.897
In this research, the activity of CaO in a CaO-SiO2-MgO-Al2O3-B2O3 blast furnace slag system at T = 1773 K was measured us-
ing the reference slag method with Sn as the metal solvent and CaF2-CaO as the reference slag. Based on the obtained experi-
mental results, the iso-activity diagram of CaO in the CaO-SiO2-MgO-20%Al2O3-2.2%B2O3 slag system at 1773 K was drawn.
The effects of R(w(CaO)/w(SiO2)) and w(MgO)/w(Al2O3) on the activities and activity coefficients of CaO were also discussed.
The results show that when w(Al2O3) = 20 %, w(MgO)/w(Al2O3) = 0.50 and w(B2O3) = 2.2 %, the activities and activity coeffi-
cients of CaO in the blast furnace slag increase with an increase in R. When w(Al2O3)=20%,R = 1.30 and w(B2O3) = 2.2 %,
the activities and activity coefficients of CaO in the blast furnace slag increase with an increase in w(MgO)/w(Al2O3).
Keywords: blast furnace (BF), activity of CaO, CaO-SiO2-MgO-Al2O3-B2O3 system, reference slag method, iso-activity con-
tours
V ~lanku avtorji opisujejo merjenje aktivnosti CaO v sistemu plav`ne `lindre CaO-SiO2-MgO-Al2O3-B2O3 pri temperaturi T =
1773 K s pomo~jo referen~ne `lindrne metode s kositrom (Sn) kot kovinskim topilom in CaF2-CaO kot referen~no `lindro. Na
osnovi dobljenih eksperimentalnih rezultatov so avtorji narisali diagram izoaktivnosti CaO v `lindrnem sistemu
CaO-SiO2-MgO-20%Al2O3-2.2%B2O3 pri 1773K. Prav tako so obravnavali vpliv razmerij R(w(CaO)/w(SiO2)) in
w(MgO)/w(Al2O3) na aktivnosti in koeficiente aktivnosti CaO. Rezultati analiz so pokazali, da je pri w(Al2O3)=2 0% ,
w(MgO)/w(Al2O3)=0,50, w(B2O3) = 2,2 %, z nara{~anjem R, nara{~ata tudi aktivnost in koeficienti aktivnosti CaO v plav`ni
`lindri. Ko je w(Al2O3)=20%,R = 1,30, w(B2O3) = 2,2 %, z nara{~anjem w(MgO)/w(Al2O3) nara{~ata aktivnost in koeficienti
aktivnosti CaO plav`ni `lindri.
Klju~ne besede: plav`, aktivnost sistema CaO, sistem CaO-SiO2-MgO-Al2O3-B2O3, referen~na `lindrna metoda, konture (linije)
izoaktivnosti
1 INTRODUCTION
Slag is one of the main by-products produced in a
metallurgical process.
1
In recent years, with the increase
in the use of high Al
2
O
3
iron ore, the Al
2
O
3
content in
blast furnace slag has sharply increased.
2–6
A high con-
tent of Al
2
O
3
in blast furnace slag has a certain impact on
the desulfurization ability of blast furnace slag. Accord-
ing to the molecular ion coexistence theory, only free
(Ca
2+
+O
2–
) and (Mg
2+
+O
2–
) ion pairs in a CaO-SiO
2
-
MgO-Al
2
O
3
quaternary blast furnace slag system have
the desulfurization ability.
7–9
The activity of each compo-
nent in a desulfurization reaction is an important factor
affecting the desulfurization ability. However, there are
relatively few studies on the activity of MgO in the blast
furnace slag with a high Al
2
O
3
content.
Many researchers also experimentally studied the ac-
tivity of silicate melt components. Hakan et al. measured
the activities of MnO in a CaO-SiO
2
-MgO-Al
2
O
3
-MnO
slag system using the gas equilibration technique at
1773 K.
10
It was found that the activities of MnO in-
crease with increasing MnO, and tend to increase with an
increasing CaO-to-MgO ratio. Hu et al. measured the ac-
tivity of Al
2
O
3
in CaO-SiO
2
-MgO-Al
2
O
3
slags at 1873 K
using the chemical equilibrium method.
11
Dong et al.
measured the thermodynamic activity of vanadium oxide
using the gas slag equilibrium technique. It was found
that the activities of vanadium(III) oxide did not cause
any significant change with an increasing temperature.
12
Many researchers such as Wen, Morita, Huang, Ohta,
Dai, Liu and Rao also studied the activity of slag.
13–20
However, the activity data for the CaO of high Al
2
O
3
melts are extremely insufficient. Therefore, this investi-
gation uses the reference slag method to experimentally
determine the activity of CaO in the CaO-SiO
2
-MgO-
Al
2
O
3
-B
2
O
3
blast furnace slag system. The biggest ad-
vantage of this method is that it does not rely on thermo-
dynamic data and can provide more accurate CaO activ-
ity experimental values.
Using Sn as the metal solvent and CaF
2
-CaO as the
reference slag, the activity of CaO in the CaO-SiO
2
-
MgO-Al
2
O
3
-B
2
O
3
blast furnace slag system was mea-
sured using the reference slag method at T = 1773 K.
The effects of R(w(CaO)/w(SiO
2
)) and w(MgO)/w(Al
2
O
3
)
on the activity of CaO were also discussed.
Materiali in tehnologije / Materials and technology 57 (2023) 6, 639–643 639
UDK 536.77:669.162.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(6)639(2023)
*Corresponding author's e-mail:
lms0612@126.com (Y. C. Guo)
2 EXPERIMENTAL PART
2.1 Principle of activity determination
Based on the basic principle of chemical equilibrium,
the activity of CaO in the CaO-SiO
2
-MgO-Al
2
O
3
-B
2
O
3
blast furnace slag system was measured. The slag-metal
equilibration technique used in this study makes use of
the following equilibrium reactions:
(CaO)
mea.
+ C(s) = [Ca]
Sn
+ CO(g) (1)
K
ap
aap
2
=
⋅
⋅⋅
[Ca]
(CaO)
CO
C
  (2)
(CaO)
ref.
+ C(s) = [Ca]
Sn
+ CO(g) (3)
K
ap
aap
4
=
⋅
⋅⋅
ref[Ca]
ref(CaO)
CO
C
  (4)
where K
2
and K
4
are the equilibrium constants, a
(CaO)
is
the CaO activity of the measured slag relative to pure
solid, a
ref(CaO)
is the CaO activity of the reference slag
relative to pure solid, a
[Ca]
is the activity of Ca in molten
Sn at equilibrium with the measured slag relative to the
infinite dilute solution, a
ref(CaO)
is the activity of Ca in
molten Sn at equilibrium with the reference slag relative
to the infinite dilute solution, p
(CO)
is the pressure of CO
and p
CO
  is the standard atmosphere of CO. When the
conditions are the same for reaction (1) and reaction (3),
the equation K
2
= K
4
can be written as follows:
aa
a
a
(CaO) ref(CaO)
[Ca]
ref[Ca]
=⋅ (5)
At a temperature of 1773 K, using Sn as the metal
flux and using the reference slag method, the experimen-
tal determination of the CaO activity can be assumed to
follow Henry’s law due to a low concentration of Ca in
Sn. When the reaction reaches equilibrium, the concen-
tration of Ca in Sn is small enough to make Henry’s law
suitable,
17
so f
[Cu]
= 1. CaO saturated slag (a
ref(CaO)
=1)is
selected as the reference slag. Equation (6) can be de-
rived from Equation (5):
a
x
x
(CaO)
[Ca]
ref[Ca]
= (6)
Therefore, when the values of x
[Ca]
and x
ref[Ca]
are de-
termined, the activity of CaO can be calculated with
Equation (6) where x
[Ca]
denotes the molar fraction of Ca
in Sn at equilibrium with the measured slag and x
ref[Ca]
denotes the molar fraction of Ca in Sn at equilibrium
with the reference slag. The activity coefficient of CaO
in slag,   (CaO)
, can also be determined with the following
equation:
  (CaO)
(CaO)
[CaO]
=
a
x
(7)
where x
[CaO]
denotes the molar fraction of CaO.
2.2 Selection of reference slag
In this study, the activity of CaO in the CaO-SiO
2
-
MgO-Al
2
O
3
-B
2
O
3
slag system was measured using a
kind of CaO saturated slag as the reference slag. Fig-
ure 1 shows the phase diagram of the CaF
2
-CaO binary
slag system.
21
It can be seen that when the mass fraction
of CaO is greater than 18 %, there is a saturated area of
CaO. Therefore, the H(w(CaF
2
)=7 0%a n dw(CaO) =
30 %) composition was selected as the reference slag.
CaF
2
volatilization causes the initial slag composition,
CaF
2
-CaO, to shift slightly to the right at point P shown
in Figure 1, but it is still in the saturated region of CaO.
Therefore, it does not affect the characteristics of the
CaF
2
-CaO slag as the reference slag.
2.3 Experimental procedure
At the beginning of the experiment, 5 g of metal Sn
particles were put into each hole of a graphite crucible,
and then 10 g of a pre-molten slag sample was put into it.
One hole was filled with the reference slag, while the
other holes were filled with the slag to be measured. The
X. WANG et al.: THERMODYNAMIC PROPERTIES OF CaO-SiO2-MgO-Al2O3-B2O3 SLAG SYSTEMS AT 1773 K
640 Materiali in tehnologije / Materials and technology 57 (2023) 6, 639–643
Figure 2: Schematic of the experimental apparatus Figure 1: CaF
2
-CaO phase diagram
21
experiment was carried out in a high-temperature tubular
furnace. In Figure 2, the experimental apparatus is
shown. The experimental temperature was controlled by
a proportional-integral-differential (PID) controller. High
purity (99.99 %) chemical reagents including CaO, SiO
2
,
Al
2
O
3
, MgO and B
2
O
3
were used to mix the slag to be
measured in accordance with the slag composition re-
quired by the experiment, as shown in Table 1. All ex-
periments were carried out at 1773 K. They were started
when the temperature of the tubular furnace sample rose
to 1773 K. Due to the uncertainty of the equilibrium time
required for the reaction to ensure that the reaction
reached equilibrium during the experimental process and
provide for the accuracy of the test results, the slag sam-
ple with a composition of (w(CaO) = 40 %, w(SiO
2
)=
33 %, w(MgO) = 4.8 %, w(Al
2
O
3
)=20%andw(B
2
O
3
)=
2.2 %) was selected. Under completely identical experi-
mental conditions, experiments were conducted at
1773 K, with reaction times of (4, 12, 18 and 26) h, re-
spectively. The Ca content in tin corresponding to the
slag samples at different reaction times is shown in Fig-
ure 3. From this figure, it can be seen that after 18 hours
of reaction, the Ca content in metal Sn remained basi-
cally unchanged. Therefore, the equilibrium time was de-
termined to be 24 h. The metal and slag were carefully
separated for chemical analyses with inductively coupled
plasma-atomic emission spectrometry (ICP-AES).
Table 1: Chemical compositions of the measured slag (mass %)
No.
Slag compositions (w/%)
R, -
w(MgO)/
w(Al
2
O
3
),-
CaO SiO2
MgO Al
2
O
3
B
2
O
3
1 34.73 33.07 10.0 20 2.2 1.05 0.50
2 36.27 31.53 10.0 20 2.2 1.15 0.50
3 37.67 30.13 10.0 20 2.2 1.25 0.50
4 38.95 28.85 10.0 20 2.2 1.35 0.50
5 41.15 31.65 5.0 20 2.2 1.30 0.25
6 40.02 30.78 7.0 20 2.2 1.30 0.35
7 38.89 29.91 9.0 20 2.2 1.30 0.45
8 37.76 29.04 11.0 20 2.2 1.30 0.55
3 RESULTS AND DISCUSSION
3.1 Activity of CaO in the CaO-SiO
2-MgO-Al
2O
3 slag
system
Using Sn as the metal solvent and CaF
2
-CaO as the
reference slag, the activity of CaO in the CaO-SiO
2
-
MgO-Al
2
O
3
-B
2
O
3
blast furnace slag system was mea-
sured with the reference slag method at T = 1773 K. Ta-
ble 2 shows the mole concentration of Ca in the Sn metal
solvent reacted with the reference slag and the activity of
CaO in the reference slag. Table 3 shows the mole frac-
tion of Ca in the Sn metal solvent reacted with the mea-
sured slag and the activity of CaO in the measured slag.
Based on the experimental results, the iso-activity dia-
gram of CaO in the CaO-SiO
2
-MgO-20%Al
2
O
3
-
2.2%Al
2
O
3
slag system at 1773 K was drawn, as shown
in Figure 4.
Table 2: Concentrations of Ca in Sn for two groups after equilibrium
and the activities of CaO in CaF
2
-CaO system
Reference
slag
x[Ca],- Average
value
a
ref[CaO]
,-
12
CaF2
-CaO 0.0260 0.0284 0.0272 1
3.2 Effect of R on the CaO activity
Figure 5 shows the effect of R on the activities and
activity coefficients of CaO in slag when w(MgO)/
w(Al
2
O
3
) = 0.50, w(Al
2
O
3
)=20%andw(B
2
O
3
) = 2.2 %.
It can be seen that when w(MgO)/w(Al
2
O
3
) = 0.50,
w(Al
2
O
3
)=20%andw(B
2
O
3
) = 2.2 %, the activities and
activity coefficients of CaO in the blast furnace slag in-
X. WANG et al.: THERMODYNAMIC PROPERTIES OF CaO-SiO2-MgO-Al2O3-B2O3 SLAG SYSTEMS AT 1773 K
Materiali in tehnologije / Materials and technology 57 (2023) 6, 639–643 641
Table 3: Compositions and experimental results for slag samples
No.
Slag composition (w/%)
R, -
w(MgO)/
w(Al
2
O
3
),-
x
[Ca]
,- a
(CaO)
,-   (CaO)
CaO SiO
2
MgO Al
2
O
3
B
2
O
3
1 34.73 33.07 10.0 20 2.2 1.05 0.50 0.002011 0.0739 0.1927
2 36.27 31.53 10.0 20 2.2 1.15 0.50 0.002239 0.0823 0.2057
3 37.67 30.13 10.0 20 2.2 1.25 0.50 0.002435 0.0895 0.2156
4 38.95 28.85 10.0 20 2.2 1.35 0.50 0.002695 0.0991 0.2311
5 41.15 31.65 5.0 20 2.2 1.30 0.25 0.002413 0.0887 0.1911
6 40.02 30.78 7.0 20 2.2 1.30 0.35 0.002458 0.0904 0.2022
7 38.89 29.91 9.0 20 2.2 1.30 0.45 0.002524 0.0928 0.2156
8 37.76 29.04 11.0 20 2.2 1.30 0.55 0.002628 0.0966 0.2334
Figure 3: Ca content in metallic Sn at different reaction times
creased with the increase in R. When w(MgO)/w(Al
2
O
3
)
= 0.50, w(Al
2
O
3
)=20%andw(B
2
O
3
) = 2.2 %, the con-
tent of CaO in the blast furnace slag increased with the
increase in R. The free oxygen ions (O
2–
) dissociated
from CaO are enough to react with SiO
2
and Al
2
O
3
in the
high-temperature melt to form silicate ions and
aluminate ions, greatly consuming acid oxides and main-
taining a relative CaO content in the slag.
3.3 Effect of w(MgO)/w(Al
2
O
3
) on the CaO activity
Figure 6 shows the effect of w(MgO)/w(Al
2
O
3
)o n
the activities and activity coefficients of CaO in slag
when R = 1.30, w(Al
2
O
3
)=20%andw(B
2
O
3
) = 2.2 %. It
can be seen that when R = 1.30, w(Al
2
O
3
)=20%and
w(B
2
O
3
) = 2.2 %, the activities and activity coefficients
of CaO in the blast furnace slag increased with the in-
crease in w(MgO)/w(Al
2
O
3
). When R = 1.30, w(Al
2
O
3
)=
20 % and w(B
2
O
3
) = 2.2 %, the content of CaO in the
blast furnace slag increased with the increase in
w(MgO)/w(Al
2
O
3
). The free oxygen ions (O
2–
) dissoci-
ated from CaO are enough to react with SiO
2
and Al
2
O
3
to form silicate ions and aluminate ions, greatly consum-
ing acid oxides.
4 CONCLUSIONS
Using Sn as the metal solvent and CaF
2
-CaO as the
reference slag, the activity of CaO in the CaO-
SiO
2
-MgO-Al
2
O
3
-B
2
O
3
blast furnace slag system was
measured using the reference slag method at T = 1773 K.
The conclusions are as follows: 1) When w(MgO)/
w(Al
2
O
3
) = 0.50, w(Al
2
O
3
)=20%andw(B
2
O
3
) = 2.2 %,
the activities and activity coefficients of CaO in the blast
furnace slag increase with the increase in R. 2) When R =
1.30, w(Al
2
O
3
)=20%andw(B
2
O
3
) = 2.2 %, the activi-
ties and activity coefficients of CaO in the blast furnace
slag increase with the increase in w(MgO)/w(Al
2
O
3
).
Funding
Scientific Research Program Funded by Education
Department of Shaanxi Provincial Government (Program
No.23JC036) and Scientific and Technological Plan Pro-
ject of Xi’an Science and Technology Bureau (Program
No. 23KGDW0031-2022).
This research was financially supported by the Xi’an
Aeronautical Polytechnic Institute (22XHZK-05) and
(115020121/76/2023XHQDJ02).
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