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COMPATIBILITY RELATIONSHIPS IN THE SYSTEM CaO.SiO2-Fe2O3-FeO
R. Canha, A. Segadães
To cite this version:
R. Canha, A. Segadães. COMPATIBILITY RELATIONSHIPS IN THE SYSTEM CaO.SiO2-Fe2O3- FeO. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-461-C1-466. �10.1051/jphyscol:1986169�.
�jpa-00225600�
JOURNAL DE PHYSIQUE
C o l l o q u e C 1 , supplement au n02, Tome 47, f h v r i e r 1986 page cl-461
COMPATIBILITY RELATIONSHIPS I N THE SYSTEM Ca0.Si02-Fe20,-Fe0
R.H. CANHA* and A.M.
SEGADAES
Universidade de Aveiro, Depart. Engenharia Cerdrnica e do V i d r o , 3800 Aveiro. Portugal
RGsmG
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L'Btude de l a c m p a t i b i l i t 6 des phases dans l e systSme CaO-SiO2- Fe203-FeO a Bt6 r&lis@edans l ' a i r e t pour un rapport molaire CaO/Si02 cons- t a n t au moyen de rn&h&.es t h ~ r a v i m ~ t r i q u e s e t c6ramgraphiques. Des donnees de diffraction des rayons-X ont St6 obtenues e t confinnent l a coexis- tence de t r o i s phases condensks dans un domaine de tenp5ratures e t de cm- positions. Dans l e s y s t h quaternaire l e liquidus isobare e s t une surface dont l'intersection avec le plan CaO/Si02 constant e s t une ligne contenue dans l e triangle de cornpsition Ca0.Si02-Fe203-Fe0.Abstract
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The study of compatibility relationships in the system CaO-SOg- Fe203-FeO was carried out i n a i r a t constant CaOjSi02 molar r a t i o using a thermobalance and the ceramographic method. Also X-ray difraction data were obtained t o confirm the coexistence of three condensed phases over a range of temperatures and compositions. In the quaternary system, the liquidus isobaric determined i n a i r is a surface and its intersection with the constant CaO/SiO2 plane is a l i n e on the composition triangle CaO.Si02-Fe203-Fe0.I
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INTRODUCTION-
I t i s too well known that the iron oxides, because of t h e i r dependence on the oxygen conditions of the atmosphere, affect the behaviour of basic refractories largely used i n the iron and certain non-ferrous smelting processes.
J u s t as iron smelting slags are based on a silica-alumina gangue with lime added as a flux, so non-ferrous smelting slags are based on a silica-iron oxide gangue and lime as a flux. During smelting, conditions within the charge change progressively, the temperature increases and the oxygen potential may decrease (oxide smelting) or in- crease (matte smelting). A t the low oxygen pressures prevailing in these operations the iron oxide i s nominally FeO. S t i l l , some f e r r i c iron must always be present. I t can, therefore, be considerable changes i n composition between i n i t i a l and f i n a l slag.
Processes involving slags are necessarily carried out i n refractory-lined vessels, and slag attack is inevitable. Attack of refractories by slag is an exceedingly com- plex process, but the study must beginswith the relevant thermal equilibrium diagram, i . e . that f o r the oxides of the refractory plus those present in the slag[l]
.
One common d i f f i c u l t y is t h a t few metallurgical slags are simple binary o r even ter- nary systems. Data on higher systems are scanty and frequently the deductions must be based on a simple system. Fortunately, i n many important cases the oxides of the simplest system make up the greatest proportion of the slag and the deductions are
*Present address: SPAL, Apt9 26, Ponte da Torre, 2461 Alcoba~a codex, Portugal
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986169
JOURNAL DE PHYSIQUE
good approximations and, f o r comparative purposes, are invaluable.
The appropriate equilibrium diagram i n t h i s case i s that f o r Ca0-SO2-Fe203-Fe0. This system, as a non-condensed system, i s a part of the quaternary system Ca-Si-Fe-0. I f the oxygen pressure i s fixed, the r a t i o ~ e ~ * / ~ e ~ + depends solely on the temperature and it is found experimentally that the reduction of ~ e 3 + increases when the tempera- t u r e rises. The composition of any mixture, with rising temperature, follows a straight l i n e away from the 0 apex of the tetrahedron. Along t h i s l i n e the ratios Ca/Si, Ca/Fe and Si/Fe remain constant, but the r a t i o s metal/oxygen change. Results by White[Z] shawed t h a t , while the additions of CaO or C2S t o liquid Fe, i n a i r a t constant temperature, increased the Fe 0 /FeO r a t i o , additions of CS affected very l i t t l e t h a t ratio. Work on the system &g8-~ 0 -CS 131 showed that a t temperatures a t which the s i l i c a t e would be t o t a l l y in the gohen s t a t e , the solid solubility of CaO and SiO a r e very small and such a s the C/S r a t i o i n the s i l i c a t e is not altered.
The s i d c a t e could therefore be considered as an independent component of the system.
Based on these findings, and also because the lime/silica molar r a t i o usually varies between 0.0 and 1.0 i n commercial basic refractories, the present work w i l l only deal with the CaO.Si0 -Fe 0 -FeO plane, and the changes i n composition of mixtures made of FeZ03+CS along tf;BirZraaction paths w i l l be followed as temperature rises.
I1
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COMPATIBILITY RELATIWSHIPS OFTHE
IRON OXIDESThe Gibbs phase rule f o r a s y s t q with P phases and C components is described by the equation (in which V i s the variance of the system) :
P + V = C + 2
A closed non-condensed system, i . e . with v o l a t i l e constituents whose p a r t i a l pressures make up the t o t a l pressure on the system, provided that the number of condensed phases is kept constant, w i l l increase its variance by one when open t o an atmosphere with inerts. In t h i s case, the t o t a l pressure on the 'system i s independent of the p a r t i a l pressures of the constituents. The change i n the variance of the system does not de- pend on the number of constituents i n the atmosphere, since t h e i r p a r t i a l pressures are not independent: the change imposed on e i t h e r one of them, keeping constant a l l the other conditions, would lead the system, by condensation o r vaporization, to the same equilibrium state. In other words, the phase rule f o r an open non-condensed sys- tem is:
P + V = C + 3
In systems containing oxides, i n a i r , and considering the oxygen equilibrium between the atmosphere and the condensed part of the system,fixing the t o t a l pressure and the oxygen p a r t i a l pressure (values which can be fixed separately) reduces the variance by two, e.g. i n a ternary system (C=3) the equilibrium of three condensed phases (two solids and one liquid) with the vapour phase (P=4) is invariant @=O).
I f the CaO.Si0 i n the system CaO-SiO -Fe 03-FeO, with a C/S r a t i o = l , can be regarded as an independkt component, then i n $he fernary system CS-Fe 0 -FeO the equilibrium of three condensed phases i n a i r would be invariant. ~ a t u r a l l g h e system w i l l be en- t i r e l y i n the liquid s t a t e before the binary CS-FeO is reached.
The binary system Fe 0 -FeO i s well known [4]., The binary system CS-Fe 0 i s a section of the ternary syst& ?a0-sio2-Fe OF Assuming that there is no s i g n i f b a n t solid solubility of CaO in the iron oxiie the isobaric section on the ternary can be sketch- ed as shown i n Figure 1.
111
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EXPERIMENTAL METHODSThe experimental procedure adopted was similar t o that followed by Willshee and White
151
in t h e i r investigation of the system Mg0-Fe203-FeO in a i r , i . e . the thermobalance was used t o follow weight changes i n f u l l y oxidized mixtures of CaO.SiOZ and Fe O3 as a function of temperature. The composition of these mixtures move along reactign paths a s they loose oxygen when the temperature rises. Isotherms were constructed by joining the compositions reached a t the particular temperature. The boundaries of the phase areas were established from the sharp changes i n direction of the isotherms.To confirm the phase boundaries established with the thermobalance, selected mixtures were pressed into p e l l e t s and f i r e d a t selected temperatures f o r sufficient time to
allow equilibrium t o be attained, and water-quenched. They were then prepared f o r ce- ramographic examination, the phases present being identified under the optical re- flected l i g h t microscope, o r f o r X-ray difraction. A molybdenumwound v e r t i c a l fur- nace was used t o f i r e the specimens enclosed i n platinum f o i l envelopes.
Figure 1
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Tentative isobaric diagram of the system Ca0.Si02-Fe 0 -FeO constructed from the relevant binary diagrams and by analogy with2tl?at of the system MgO-Fe 03-FeO[s].
Dash-dotted lines a r e a sketch of the liquidus surface of tge system. Dotted straight l i n e s a r e reaction paths.IV
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RESULTS AND DISCUSSIWThe dissociation curves obtained f o r t h e samples investigated are shown i n Figure 2 and they suggest the following comments:
a) In sample A.l (Fig.2-A), containing only Fe 0
,
the transition temperature from hematite t o spinel (magnetite), refered by ~il$sl?ee and White151
t o be 1388QC(sudden loss of oxygen) was found t o occur a t 1375PC. Also there i s an i n i t i a l loss of oxygen increasing slowly as temperature approaches t h a t value.
b) Samples M. 1 t o M. 7 (Fig. 2-A and 2-B) produce similar dissociation curves, with a marked oxygen loss over a narrow temperature range which could correspond t o an in- variant reaction (coexistence of hematite, magnetite and liquid) i f the system is t o be a ternary one. To check these r e s u l t s , new dissociation curves from two samples of t h i s group (M.5 and M.6) were obtained under more accurate experimental conditions, namely, the heating r a t e was reduced t o 1/6 of t h e previous one. The new dissociation curves show clearly that the reaction i s not invariant and therefore the system should be a four-component one. The shape of the former curves must be due t o a delaied equi- librium, because of the higher heating r a t e (6Wmin).
c) Figures 2-C and 2-D show two groups of dissociation curves quite different from the previous ones, with smooth but large oxygen losses, as temperature rises. Again, new dissociation curves from one sample i n each group (M.10 and M.13) were obtained under a slower heating r a t e , but the new curves show the same tendency, although smoother (better equilibrium). I t should be noted that between samples M.12 and M.13 the r a t e of oxygen loss decreases suddenly, increasing again a f t e r sample M.13. Hence
C1-464 JOURNAL DE PHYSIQUE
there will be a sharp
chane in the direction of the isotherms between these two samples, to which correspon%s a boundary between two different phase areas.
Figure 3 shows the isobaric diagram constructed from the dissociation curves, compo- sitions being expressed as weight percentages of Fe203, FeO and CS. Compositions of starting mixtures, fully oxidized, lie along the Fe203 CS edge of the composition tri-
Figure 2 - Dissociation curves i n air of Fe OZ+Ca0.Si02 mixtures (compositions as shown, heating rage 6~~/min) .
Dashed curves represent experiments carried out under improved equilibrium conditions (heating rate lQC/min) .
angle. The dotted straight lines starting on this edge represent reaction paths along which the compositions of the experimental mixtures would change with changes i n oxy- gen content. The isotherms were constructed by joining the compositions reached at the corresponding temperature. The boundaries of phase areas were established from the sharp changes in direction of the isotherms and the compatible phases in each area, identified under the microscope.
Figure
3also shows the tentative temperature/composition diagram of the system Fe203-
-CaO.SiO in air, as constructed from the results of the quenching experiments. In
this diagram, it is very clear that three condensed phases,
viz. hanatite,wollastoniteand liquid coexist over a range of temperatures. X-ray difractograms of samples M.10
andM.11, fired at 1250?C confirmed this finding. The non-invariant coexistence of three condensed phases implies that the system has more than three components.In other words,
CSis not an independent component of the system which is then a four-component one. Any other three condensed phase-equilibrium in the system can not be invariant and the coexistence of hematite, magnetite and liquid must also occur over a range of temperatures, however narrow, as suggested
bythe dissociation curves of samples
M . 1to M. 7.
Figure
3- Isobaric diagram of the seftion Ca0.Si02-Fe203-Fe0 in air of
the system CaO-Fe 03-FeO-SiOZ and related temperaturelcomposition diagram
of the binery sys$em Fe203-CaO.Si0 as constructed from dissociation
curves and quenching experiments. &tted straight lines are reaction paths
along which the compositions of starting mixtures change with changes in
oxygen content. Thin lines are isotherms.
JOURNAL DE PHYSIQUE
The tentative position of the eutectic in the binary diagram corresponds to the com- position range between samples M.12 and M.13, where a sharp change in the direction of the isotherms was detected. In this diagram, the phase areas corresponding to spi- nel, hematite, spinel+hematite and spinel+hematite+liquid were not accurately deter- mined and they were drawn based on the reasoning above.
In the quaternary system, the intersection of the liquidus isobaric surface, i n air
(Ptotal=~.l m a , ~ 0 ~ = 0 . 0 2 1 ma),
with the C/S=l plane is a line which lies on the composition triangle CaO.Si0 -Fe
0-FeO. The compositions of the starting mixtures and those of all the liquid phas& a?sa belong to this triangle.
V - CCNCLUSIONS
The main feature to note is that, due to some solid solution among constituents, lime and silica behave as independent components of the system which has, therefore, to be delt with as a quaternary system, to conform with the phase rule. Regions of coexist- ence of three condensed phases originally expected to occur at constant temperature, were found to extend over a range of temperatures, which eliminates the ternary sys- tem hypothesis. Although the gathered data are somehow limited, the proposed isobaric diagram should not be too different from a more accurate version.
REFERENCES
/1/ Taylor,J., "Refractory Materia1s:a series of
monographs",Vol.6-11,chapter6, ed.
A.M.Alper, Academic Press, inc., N.York, 1970
/ 2 /
White,J., The Iron
&Steel Inst., 27 (1) 1938
/3/SegadZes,A.M., Ph.D. Thesis, Univ. Sheffield, 1982
/4/
Muan,A. , Osborn,E.F. , "Phase Equilibrium among Oxides in Steelmaking",Addison- -Wesley pub. Camp., inc., N.York, 1965
/ 5 /
