liquidus surface of feo-fe2o3-sio2-cao slags at constant co2/co ratios
DESCRIPTION
Liquidus surface of FeO-Fe2O3-SiO2-CaO slags is an important parameter in various smelting and converting processes. It helps not only to optimize the slag chemistry of current processes and their fluxing strategies but also to determine the availability of new slags for more advanced technologies. In our previous publications, the liquidus surface of some multicomponent iron oxide slags has been quantified at several constant oxygen potentials and the effect of the latter, ignored until that moment, was quantified along with the effect of some minor components. In this work, the liquidus surface of some iron oxide slags is quantified at constant CO2 /CO ratios. This is a new convenient way for the quantitative description of the slag liquidus surface and the effect of several fluxes, especially in those processes, such as slag solidification, where oxygen potential changes continuously. This type of diagram also describes more dynamically the effect of oxygen potential, clarifies the relation between CO2 /CO ratio and oxygen potential in terms of the liquidus surface (not widely understood by metallurgists today) and reduces the gap between laboratory work and industrial experience.TRANSCRIPT
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Liquidus Surface of FeO-Fe2O3-SiO2-CaO Slags at Constant CO2/CO Ratios
Florian Kongoli1* and Akira Yazawa2
1FLOGEN Technologies Inc., www.flogen.com, 5757 Decelles Ave., Suite 511, Montreal, Quebec, H3S 2C3, Canada
2Tohoku University, Sendai 981-0934 Japan
Liquidus surface of FeO-Fe2O3-SiO2-CaO slags is an important parameter in various smelting and converting processes. It helps not only
to optimize the slag chemistry of current processes and their fluxing strategies but also to determine the availability of new slags for more
advanced technologies. In our previous publications, the liquidus surface of some multicomponentiron oxide slags has been quantified at several
constant oxygen potentials and the eff ect of the latter, ignored until that moment, was quantified along with the eff ect of some minor
components. In this work, the liquidus surface of someiron oxide slags is quantified at constant CO2/COratios. This is a new convenientway for
the quantitative description of the slag liquidus surface and the e ff ect of several fluxes, especially in those processes, such as slag solidification,
where oxygen potential changes continuously. This type of diagram also describes more dynamically the eff ect of oxygen potential, clarifies the
relation between CO2/CO ratio and oxygen potential in terms of the liquidus surface (not widely understood by metallurgists today) and reduces
the gap between laboratory work and industrial experience.
(Received June 30, 2003; Accepted August 14, 2003)
Keywords: liquidus, FeOx-SiO2 based slag, iron oxides, oxygen potential, CO2/CO, slag solidification, smelting, converting
1. Introduction
Iron oxide slags are the most commonly used slags in
sulfide smelting and steel making. They usually contain silica
and lime as well as other minor oxides, which are introduced
through raw materials, fluxes, dissolved refractories etc.
Liquidus surface of these slags constitutes an important
parameter for the sulfide smelting and converting processes.
It helps not only to optimize the slag chemistry of the currentprocesses and the fluxing strategies, but also to determine the
availability of new slags for more advanced technologies.
In our previous work the liquidus surface of some iron
oxide slags has been quantified at low oxygen potentials,
characteristic of reductive processes1–3)
and at intermediate
oxygen potentials, characteristic of oxidative processes4–7)
such as direct smelting and continuous converting. This was
carried out by the means of a new type of multicomponent
phase diagrams1) at constant oxygen potentials and deducted
from the use of a new thermophysicochemical model.
Through a series of these diagrams, the important eff ect of
oxygen potential on the liquidus surface of multicomponentslags, ignored until that moment, was quantified along with
the eff ect of some minor components. Considerable con-
fusion found in literature about the eff ect of some minor
components was also clarified. Among others, it was found
that this eff ect could be fundamentally diff erent in reductive
and oxidative conditions. However, confusion still exists,
especially for those particular processes in which oxygen
potential changes dynamically mainly as a result of the
continuous cooling of the slag and the use of coke in the
process. An example of these processes is the settling phase
of matte smelting which is a subsequent sub-process of matte
oxidative smelting and/or slag solidification in which
temperature drops continuously from around 1573 to
1423 K and sometimes coke breeze is used in the last stage.
In these processes, as well as in some others, contradictions
are often found between the microscopic results of the
laboratory quenching measurements and slowly cooled
solidified smelting slags from the industrial practice. Contra-
dictory assertions are also given about the eff ect of minor
components in these processes. It seems that the sensitivity of
the slag liquidus temperature toward changes of the oxygen
potentials has been ignored and this is believed to be the
reason of the above confusion.Following a previous proposal,8) the purpose of this work
is to quantitatively describe the eff ect of the dynamic changes
of oxygen potential at these particular processes through a
new type of phase diagrams at constant CO2/CO ratios,
based on the previous model. This will not only help clarify
the above confusion but will also shed light in the under-
standing of the slag solidification process and the solidified
slag mineralogy, which are recently becoming important in
the environmental point of view.
The partial pressures of oxygen throughout the article are
given as dimensionless ones defined by pO2 = (PO2
)/
(101325 Pa).
2. Variation of PO2 and CO2/CO during Slow Cooling
As stated in previous work 4–7)
oxygen potentials during
matte smelting and blister making converting at 1573 K can
be respectively approximated as 108 and 106. This is
illustrated in Fig. 1, which gives the calculated equilibrium
oxygen and sulfur pressures during oxidative copper smelt-
ing. However, the oxygen potentials of the slag might drop
below 109 during matte smelting near the solidification
temperature or in the reductive slag-cleaning furnace. The
gradual cooling at a lower temperature and the use of coke
breeze during settling change continuously the oxygen
potential. Based on the previous article8)
the variations of
pO2 and CO2/CO during cooling are given below in Figs. 2
to 4.*Corresponding author, E-mail: [email protected]
Materials Transactions, Vol. 44, No. 10 (2003) pp. 2130 to 2135#2003 The Japan Institute of Metals
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Figure 2 describes the variation of pO2 with temperature
when CO2/CO is kept constant and the variation of CO2/CO
with temperature when pO2 is kept constant. It can be seen
that a constant CO2/CO ratio describes more concisely the
continuous change of oxygen potential during equilibrium
cooling since it includes in itself many variations of pO2
depending on temperature.
Figure 3 describes the variation of the activity of FeO(l)
with temperature at constant pO2 and CO2/CO. It can be seen
that the eff ect of temperature on the activity of FeO(l) is
much less pronounced at constant CO2/CO ratio compared to
constant pO2 ratio.
Figure 4 describes the variation of pO2 and CO2/CO ratio
according to two separate equilibrium reactions and during
the equilibrium cooling of a slag ‘‘X’’ of composition
46.3 mass% FeO, 6.7 mass% Fe2O3, 37mass% SiO2 and
10 mass% CaO from 1623 K to 1373 K. It is shown that while
oxygen potential changes considerably during cooling from
around 107 to 1010, the resulting CO2/CO ratio is almost
constant. Taking into account this fact, the phase diagrams at
constant CO2/CO ratio seem to be an interesting new
alternative in the quantification of the liquidus surface of a
multicomponent slag at those processes such the continuous
cooling and solidification where the oxygen potentials
change dynamically.
Some examples of this new type of diagrams at constant
CO2/CO are given below. They have been constructed by
FLOGENTM software9)
through a new thermophysicochem-
ical model,4) which was already verified against all available
experimental data on liquidus temperatures as well as other
thermodynamic properties at several oxygen potentials.
Fig. 1 Variations of equilibrium oxygen and sulfur pressures during
oxidative matte smelting and converting. 1000 moles CuFeS2 concentrate
is assumed to be oxidized with air or oxygen enriched air at 1473.15 K and
1573.15K. ( : 1573 K Air blow; -- - - - - 1573K, 40%O2 blow; –– – –– –1473 K, Air blow).
Fig. 2 Relation between PO2 a nd CO2/CO ratio according to
2CO(g)+O2(g) = 2CO2(g) at constant CO2/CO (dashed lines) and
constant PO2 (solid lines).
Fig. 4 Variation of pO2 and CO2/CO ratio with temperature according to
3FeO(l)+O2(g) = Fe3O4(s) and 3FeO(l)+CO2(g) = Fe3O4(s)+ CO(g) at
a constant aFeO(l) and during the equilibrium cooling of a real slag ‘‘X’’ of
composition 46.3mass% FeO, 6.7mass% Fe2O3, 37mass% SiO2 and
10 mass% CaO from 1623 to 1373K.
Fig. 3 Eff ect of temperature on the activity of FeO(l) coexisting with
Fe3O4(s) at fixed PO2 (dashed curves) according to 3FeO(l)+O2(g) =
Fe3O4(s) and at fixed CO2/CO (solid curves) according to
3FeO(l)+CO2(g) = Fe3O4(s)+CO(g).
Liquidus Surface of FeO-Fe2O3-SiO2-CaO Slags at Constant CO2/CO Ratios 2131
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3. Polythermal Projection Diagrams at Constant CO2/
CO Ratio
Figure 5 presents the liquidus surface of FeO-Fe2O3-SiO2-
CaO slag at a constant ratio of log(CO2/CO) = 2 by the
means of a new format of multicomponent phase diagrams
whose basis has been previously described.1)
This is in fact
the slag liquidus surface during slow equilibrium cooling
from 1623 to 1373 K where the oxygen potentials change
continuously from about 105 to around 108. It can be
seen that at these conditions several primary phases are
present, i.e., magnetite (spinel), alpha-Ca2SiO4, Ca3Si2O7,
wollastonite/pseudo-wollastonite and silica and each of themhas its own characteristics. Some important points can be
easily made from this diagram. First, during continuous
cooling of these slags, the ‘‘magnetite’’ spinel phase is the
dominant primary precipitate phase. There is no stable
olivine or ‘‘fayalite’’ at these conditions, which suggest that
the name ‘‘fayalite slag’’ is not adequate at this particular
case. A meaningful name would be ‘‘magnetite’’ or ‘spinel’
slag if the primary precipitate phase is to be used to name the
slag. At these conditions the mineralogy of a slow cooled
solidified slag with an overall liquid composition of Fe/
SiO2=1.1 and CaO=10 mass% (point X in the diagram)
would mostly contain primary magnetite. Second, lime doesnot decrease, but instead, increases the liquidus temperature
of the slag at spinel saturation area and consequently
increases the risk of the magnetite (spinel) precipitation
which makes lime not a good flux in terms of the liquidus
temperature. This eff ect is much more pronounced in this
kind of diagrams compared to the diagrams at constant pO2
presented previously4)
and this reflects the sensitivity of the
liquidus temperature toward changes on the oxygen poten-
tials. It should be mentioned however that lime decreases the
liquidus temperature only in the region of newly proposed
FCS slag5,6) where it may be used as a good flux. Third, at
constant CaO an increase of Fe/SiO2 ratio in the ‘magnetite’
saturation area would increase the risk of ‘magnetite’precipitation. This eff ect is more pronounced in this diagram
compared to the one given at constant pO2, which again
reflects the sensitivity of the liquidus temperature toward
changes on the oxygen potentials.
Figure 6 presents the liquidus surface of the same slag at
log(CO2/CO)=1. Again this represents the liquidus surface
of this slag during continuous cooling from 1623 to 1373 K
where the oxygen potentials change continuously from about
107 to around 1010. It can be seen that besides the 5
primary phases mentioned above, two new stable phases are
present at these particular conditions i.e. olivine and wustite.
Contrary to the previous case, lime in small and limited
amounts decreases the slag liquidus temperature in the
olivine saturation area but increases it in the spinel(magne-
tite), wustite, and wollastonite saturation areas. At theseconditions the mineralogy of a slow cooled solidified slag
with an overall liquid composition of Fe/SiO2=1.1 and
CaO=10 mass% (point X in the diagram) would mostly
contain primary olivine.
Figure 7 gives the liquidus temperature of the slag at
log(CO2/CO)=0.3 which corresponds to the continuous
equilibrium cooling of the slag from 1623 to 1373 K where
the oxygen potentials drop continuously from about 108 to
around 1012. In this case spinel is not anymore a primary
Fig. 5 Liquidus surface of FeO-Fe2O3-SiO2-CaO slag at log(CO2/
CO)= 2.
Fig. 6 Liquidus surface of FeO-Fe2O3-SiO2-CaO slag at log(CO2/
CO)= 1.
Fig. 7 Liquidus surface of FeO-Fe2O3-SiO2-CaO slag at log(CO2/
CO)= 0.3.
2132 F. Kongoli and A. Yazawa
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stable phase, but all other primary phases are present i.e.
olivine, wustite, alpha-Ca2SiO4, Ca3Si2O7, wollastonite/
pseudo-wollastonite and silica. It can be seen that lime
decreases the liquidus temperature of these slags in the
olivine saturation area but increases it in almost all other
areas. At these conditions the mineralogy of a slow cooled
solidified slag with an overall liquid composition of Fe/
SiO2=1.1 and CaO=10 mass% (point X in the diagram)
would mostly contain primary olivine.
4. Isothermal Diagrams at Constant CO2/CO Ratio
Figure 8 describes the eff ect of CO2/CO ratio on the liquidregions of the current slag at 1448 K. The liquid regions for
constant pO2 have also been given for comparison. It can be
seen that, as expected, decreasing the CO2/CO ratio or the
pO2 increases the liquid region at this particular temperature
especially in the olivine, wustite and spinel surface. The slag
‘‘X’’ with an overall liquid composition of Fe/SiO2=1.1 and
CaO=10mass% would be completely liquid only at
log(CO2/CO) of 1 and 0.3 as well as at pO2 of 1010.
Figure 9 also describes the eff ect of CO2/CO ratio on the
liquid regions of the current slag at 1498 K. The liquid
regions for constant pO2 have also been given for compar-
ison. In this case also, a decrease of CO2/CO ratio or pO2
increases the liquid region. The slag ‘‘X’’ with an overall
liquid composition of Fe/SiO2=1.1 and CaO=10 mass%
would be completely liquid only at log(CO2/CO) of 1 and 0.3
as well as at pO2 of 108 and 1010. It should also noted that
the liquidus curve at log(CO2/CO)=0.3 coincides with the
one at PO2 ¼ 1010 since at this temperature these values
correspond to each other. This can be easily understood from
Table 1 that gives the relationship between the log(pO2)
values at several constant log(CO2/CO) constant ratios atvarious temperatures for the reaction 2CO2(g) =
2CO(g)+O2(g). In our particular case at log(CO2/CO) of
0.3 the corresponding value of oxygen potential is log(pO2) of
10:06. This explains the overlapping of liquidus curves at
both above-mentioned conditions.
5. Quenching and Slow Cooling
As mentioned above, contradictions are often found
among the microscopic results of laboratory quenching
measurements and slowly cooled solidified smelting slags
from industrial practice. For instance, in matte smelting,quenching experimental measurements show that ‘magnetite’
is normally the primary precipitate solid phase of the
quenched samples within the glass phase (the finely crystal-
line structure, representing frozen liquid). However, micro-
scopic examinations of relatively big amounts of solidified
slag from this process, especially from settling stage, show
olivine as the dominant crystallized phase. The fluxing eff ect
of lime in both cases is also a subject of confusion. These
disagreements can now be explained and clarified in the light
of the present work.
Figure 10 gives the liquidus temperature of the FeO-
Fe2O3-SiO2-CaO slag at Fe/SiO2=1.1 and at diff erent
constant values of CO2/CO and pO2 as well as at ironsaturation. If the above-mentioned slag ‘‘X’’ (Fe/SiO2=1.1,
CaO=10 mass%) is kept at constant oxygen potentials, as it is
almost the case of matte smelting and continuous converting
Fig. 8 Liquidus regions of FeO-Fe2O3-SiO2-CaO slag at 1448K and at
various constant values of CO2/CO and oxygen potentials.
Fig. 9 Liquidus regions of FeO-Fe2O3-SiO2-CaO slag at 1498K and at
various constant values of CO2/CO and oxygen potentials.
Table 1 The relationship between logðpO2Þ and log(CO2/CO) ratios at
various temperatures for the reaction 2CO2(g) = 2CO(g)+O2(g)
T /K DeltaH DeltaS DeltaG K Log (K) Log(PO2)
(kJ) (J/K) (kJ) Log(CO2/CO)
2 1 0.3
1373 562.100 171.038 327.239 3.61E-13 12:44 8:44 10:44 11:84
1398 561.864 170.868 322.965 8.71E-13 12:06 8:06 10:06 11:46
1423 561.626 170.699 318.696 2.04E-12 11:69 7:69 9:69 11:09
1448 561.385 170.532 314.430 4.62E-12 11:34 7:34 9:34 10:74
1473 561.143 170.366 310.169 1.02E-11 10:99 6:99 8:99 10:39
1498 560.899 170.201 305.912 2.19E-11 10:66 6:66 8:66 10:06
1523 560.653 170.038 301.659 4.58E-11 10:34 6:34 8:34 9:74
1548 560.405 169.877 297.410 9.36E-11 10:03 6:03 8:03 9:43
1573 560.155 169.717 293.165 1.87E-10 9:73 5:73 7:73 9:13
1598 559.904 169.559 288.924 3.65E-10 9:44 5:44 7:44 8:84
1623 559.652 169.402 284.687 6.99E-10 9:16 5:16 7:16 8:56
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(108 or 106) the primary precipitate phase is ‘‘magnetite’’
(spinel) within the glassy phase representing liquid. If the
slag ‘‘X’’ is equilibrated at 1433 K and pO2 ¼ 108 and then
quenched, the microscopic examination will reveal only
‘magnetite’(spinel) as a precipitate phase besides the glass. In
this case lime is not a good flux in terms of liquidus
temperature since it increases it and consequently increases
the risk of magnetite precipitation. If the same slag ‘‘X’’ is
slow cooled, as it is the case of certain relatively big amount
of slags in the industrial practice or in the settling phase of
matte smelting, the primary precipitate phase would now be
olivine. As it can be seen in the upper part of Fig. 4, duringthe equilibrium cooling of the slag X, log(CO2/CO) stays
almost constant around the value of 1 and at these conditions
the primary precipitate phase is olivine, as shown in Fig. 10.
Although the slow cooling of the industrial slag and it
solidification is not a truly equilibrium process the diagrams
at constant CO2/CO ratio are the best approximation of these
processes. The microscopic examination of many solidified
slags confirms this conclusion since it reveals that many of
these slags consist of mainly olivine. In this case lime in
limited amounts would be a good flux in terms of the liquidus
temperature, especially in the settling phase of matte
smelting where the temperature may reach 1473 K andsometimes coke breeze is used in the process. In Fig. 10 it is
also worth noting that at iron saturation the primary
precipitate phase is still olivine although at this particular
conditions oxygen potential stays almost constant around the
value of 1011 or 1012. Quenching of the slag X in iron
crucible from a holding temperature of 1360 K would
produce only olivine as primary precipitate crystals within
the field of frozen liquid.
In this light, it can be said that there is a fundamental
diff erence between quenching and slow equilibrium cooling
of an iron oxide slag at intermediate oxygen potentials. The
diff erence could be in the primary precipitate phases, on the
eff ect of minor components, in the mineralogical composi-
tion of the solidified slag, etc. This diff erence has not been
always understood and one of the reasons for that is that the
experiments have always been carried out in equilibrium with
metallic iron and in air where the oxygen potential does not
change (in air) or change only slightly (in equilibrium with
metallic iron). This is clearly reflected on the diff erence that
exists between the diagrams at constant oxygen potentials4)
and those at constant CO2/CO, as given in this work.
6. Conclusions
The liquidus surface of FeO-Fe2O3-SiO2-CaO slag was
quantified through a new type of phase diagrams at constant
CO2/CO ratios. It was shown that this is a convenient
quantitative way for the description of the slag liquidus
surface and the eff ect of minor components in those
processes, such as slag solidification, where the oxygen
potential changes continuously. It describes more dynam-
ically the eff ect of oxygen potential and clarifies the relation
between CO2/CO ratio and oxygen potential in terms of the
liquidus surface. The analysis of the variation of pO2 and
CO2/CO showed that during slow equilibrium coolingalthough the oxygen potential changes continuously, CO2/
CO ratio stays almost constant. Consequently, this new type
of diagrams at constant CO2/CO ratio also simulates the slow
cooling process of the industrial slag.
The eff ect of CO2/CO ratios and oxygen potentials on the
liquidus temperature were also quantified and the contra-
dictions often found between the microscopic results of
laboratory quenching measurements and slowly cooled
solidified smelting slags from industrial practice were
clarified. It was shown that there is a fundamental diff erence
between the quenching and slow equilibrium cooling of an
iron oxide slag at intermediate oxygen potentials in terms of
the primary precipitate phases, the eff ect of minor compo-nents, the mineralogical composition of the solidified slag,
etc. The eff ect of lime on the liquidus temperature was also
quantified at both processes for FeO-Fe2O3-SiO2-CaO
system. However, the presence of other minor components
may alter this eff ect. The constructed diagrams shed light in
the understanding of the slag solidification process and the
solidified slag mineralogy, which are recently becoming
important in the environmental point of view. The diagrams
at constant CO2/CO and pO2 are complements of each other
and both help in the quantification of the liquidus surface of
slags in several metallurgical processes.
Acknowledgment
The authors wish to thank Mitsubishi Materials Corpo-
ration and Sumitomo Metal Mining Co. Ltd. for the financial
support.
REFERENCES
1) F. Kongoli and I. McBow: EPD Congress 2000, ed. by P. Taylor, (The
Minerals, Metals and Materials Society, Warrendale, PA, 2000) pp. 97-
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2) F. Kongoli and I. McBow: Copper 99, Vol. VI: Smelting, Technology
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Landolt and T. Utigard, (The Minerals, Metals and Materials Society,
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3) F. Kongoli, M. Kozlowski, R. A. Berryman and N. M. Stubina: James M.
Toguri Symposium on the Fundamentals of Metallurgical Processing,
Fig. 10 Liquidus temperature of the FeO-Fe2O3-SiO2-CaO slag at Fe/
SiO2=1.1 and at diff erent constant values of CO2/CO and pO2 and at iron
saturation.
2134 F. Kongoli and A. Yazawa
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ed. by G. Kaiura, C. Pickles, T. Utigard and A. Vahed, (The Canadian
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and Petroleum, Ottawa, 2000) pp 365-377.
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Conference on Molten Slags, Fluxes and Salts, Stockholm, Sweden -
Helsinki, Finland, 2000, ed. by S. Seetharamanand D. Sichen, (Trita Met
85, 2000), CD-Rom, 016.pdf.
8) A. Yazawa: Tetsu-to-Hagane 86 (2000), 1-10.
9) F. Kongoli, I. McBow and S. Llubani: FLOGEN Technologies Inc.
(www.flogen.com), 1999-2003.
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