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PHASE EQUILIBRIA IN THE SYSTEM MgO-Mn2O3-MnO-CaSiO3 IN AIR
V. Oliveira, N. Brett
To cite this version:
V. Oliveira, N. Brett. PHASE EQUILIBRIA IN THE SYSTEM MgO-Mn2O3-MnO-CaSiO3 IN AIR.
Journal de Physique Colloques, 1986, 47 (C1), pp.C1-453-C1-459. �10.1051/jphyscol:1986168�. �jpa-
00225599�
JOURNAL DE PHYSIQUE
Colloque C1, supplement au n02, Tome 47, f6vrier 1986 page c1-453
PHASE EQUILIBRIA IN THE SYSTEM MgO-Mn,03-MnO-CaSiO, IN AIR
V.A.G. OLIVEIRA'~) and N.H. BRETT
Department of Ceramics, Glasses and Polymers, The university of Sheffield, Elmfield, Northumberland Road,
GB-Sheffield S10 2TZ, U.K.
RbsumG
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Le systsme Mg0-Mn203-Mn0 prgsente dans l'ai'r deux familles de solutions solides, (Mg, Mn)Mn204 et (Mg, Mn)O, dont les limites de phases ont 6tb pr6cis6es. A 160025°C, il existe un domaine triangulaire dans lequel les deux solutions solides et une phase liquide coexistent avec une phase gazeuse. Le rapport ~ n 3 + / ~ n ~ + de la phase cristalline est abais- s6 par addition de CaSiOg; ceci entraine une stabilisation de la phase (Mg, Mn)O 1 plus basses temperatures.Abstract
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The Mg0-Mn203-Mn0 system in air exhibits two series of solid solu- tions, (Mg,Mn)Mn204 and (Mg,Mn)O, the boundaries of which have been located.At 1600 +5' C a tie-triangle exists in which these two solid solution phases and a liquid phase coexist with the gas phase. The Mn3+/Mn2+ ratio of the crystalline phases is lowered by the addition of CaSi03 and this has the effect of stabilising the (Mg,Mn)O phase at lower temperatures.
I
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INTRODUCTIONMagnesia (MgO) refractories have increased in importance in recent years mainly because of their high refractoriness, resistance to iron oxide attack, alkalies and high lime fluxes. Natural magnesite (MgC03) is still a major source of raw material for magnesia-based refractories despite the increasing production of sea-water and brine well magnesias of high purity. One of the largest deposits of refractory grade magnesite is reported to be in Brazil /1,2/ where different grades of dead-burned magnesias of low impurity content are produced by enriching the run-of-mine ore
followed by a single or double firing process. One of the impurities present in these natural magnesites is MnO (up to % 0.8 wt.%) and in this paper we describe the re- sults of a study of phase equilibria in the Mg0-Mn203-Mn0 system and the effect of small additions of CaSi03 (CS) on that system. The phases formed depend ontheoxida- tion state of the transition metal which is a function of temperature and oxygen par- tial pressure (Po2). The work was carried out under constant Po2 equal to 0.021 MPa by opening the system to air.
I1
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LITERATUREIn the Yi-0 system the oxides m 0 2 , ~ n ~ o j , MngO4 and MnO exist as stable phases and can be converted from one form to another by carefulchoice of oxygen partial pres- sure and temperature /1-5/. The region Mn20-3-MnO is of principal interest to this study and brief mention of results reported in that region will be made. MnpOj is reported to covert in air to Mn304 at 877'~ /3/ and 871°c /1/ but the conversion back to Mn203 was found to be much slower than the reduction reaction. The melting
(1) Magnesita S.A., 30.000 Belo Horizonte
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MG, Brazil.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986168
JOURNAL DE PHYSIQUE
point of Mn3O4 in air was found to be 1567 f 4 " ~ and an invariant situation occurred at 1540'~ where Mng04, MnO and liquid were in equilibrium with a gas phase of 0.010 MPa Pop /3/. Later, the same invariant point was reported at 1546'~ and at the compo-
sition 85.1 molar % MnO, 14.9 molar % Mn2Og and oxygen pressure of 0.007 MPa 141.
Other workers reported the melting point of PI304 at 1 5 5 5 ~ ~ at approximately 0.010 MPa Po? and found the oxide to be nearly stoichiometric up to 0.101 MPa Po,.
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- c The oxide MnO exhibits positive deviations from stoichiometry and forms an extensive region of solid solution (MnO ) within the system.ss
In the system MgO-manganese oxide in air an invariant point was found at 1587 +loOc where MgO-MnO solid solution, MgMn204 spinel and liquid of approximate composition 1.75 molar % MgO, 98.25 molar % MnO were in equilibrium with the gas phase /6/. The MgO-MnO solid solution phase field is extensive and domjnates the liquidus surface.
The systems Mn0-Ca0-Si02 /7/, Mn0-Mg0-Si02 /8/ and Mn0-Mg0-Ca0 /9/ have been studied under low oxygen partial pressures where manganese is present in the divalent state.
The ~gO-Ca0-Si0~ system has been extensively studied and revised /lo/.
111
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EXPERIMENTALThe starting materials were BDH Mg(OH)2 calcined at 900'~ for 15 h in air and BDH manganese dioxide calcined at 800'~ for 15 h in air to give MnpOg. Pre-formed CaSiOg was made by calcining the appropriate amounts of BDH Analar grade CaC03 and high purity acid washed Belgian sand (99.8% SiOp) at 950'~ overnight, then pressing into pellets which were fired in air at 1400Oc for 15 h, ground, repressed and fired at 1480'~ for 18 h. The materials were ground to pass 300 mesh B.S. sieve and stored in air-tight bottles in a desiccator until used. Samples for thermobalance runs were prepared by weighing the desired amounts of MnpOg, MgO and CaSi03, mixing under ace- tone, drying at 110'~ then pressing into pellets weighing 2 to 3 g which were con- tained in open Pt crucibles. The first stage of the firings was carried out in a Stanton automatic thermobalance Model HT-SF and the samples were equilibrated at 700'~ to give a base weight for subsequent calculation of weight loss then raised
progressively to 1300'~. After cooling the furnace to room temperature the samples were transferred toatnolybdenum-wound thermobalance, raised to 1300'~ and equili- brated at that temperature using this weight as a reference for subsequent weight losses at higher temperatures up to 1700'~ which were monitored at 25'~ temperature intervals. Observed weight changes (f 2.l0-~ kg) were corrected for buoyancy effects and attributed to pick-up or loss of oxygen by the transition metal oxide. Selected samples, already analysed in the thermobalance, were quenched to room temperature in a stream of cold air or in water from higher temperatures and analysed by reflected light microscopy, x-ray diffraction and electron microprobe analysis to identify the phases present.
IV
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RESULTS AND DISCUSSIONIV.l The system Mg0-Mn203-Mn0 in air
A number of compositions were examined that ranged from 0 to 72.5 molar % MgO in mix- tures of MgO-Mn203. From thermobalance data oxygen to manganese atomic ratio (O/Mn) was plotted against temperature, typical curves are shown in Figure 1. Sample TB2, pure MnpOg, lost oxygen isothermally at 8 7 0 ~ ~ forming Mn304, thereafter a small but gradual change of composition occurred up to 1570'~ when the onset of melting was noted. The data were replotted to show isotherms in the ternary system and the dis- sociation paths (dashed lines) of the compositions examined. These results, together with those obtained by XRD, EPMA and reflected light microscopy on quenched samples, allow the equilibrium phase diagram for the system Mg0-Mn203-Mn0 in air to be drawn, Figure 2. In this diagram the region limited by the oxides w-1~03, Mng04 and MgMnp04 is the stability field where Mn2Og is in equilibrium with (Mg,Mn)Mn204 spinel. Mn2Og is in equilibrium with Mng04 at 870'~ and the addition of MgO lowers the temperature at which spinel is formed. The region marked S on the diagram is the stability field of the continuous solid solution series between MgM1-1204 and Mnj01, ie. (Mg,Mn)Mn201+
spinel. A large two-phase field of spinel and (Mg,Mn) exists (P+S) and at higher temperatures and higher MgO contents the (~g,Mn)o phase is stable. The small region 1-2-5-4 is the phase field where (Mg,Mn)Mn204 spinel is in equilibrium with liquid;
0
800 800 1000 1100 1200 1300 14W 1500 1600 1700
Fig. 1
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O/Mn ratio versus temperature for various compositions.point 5 is hypothetical and was not determined. A tie triangle 1-2-3 exists at 1600'~ where spinel, (Plg,Mn)O and liquid coexist with gas phase (air) in a mono- variant situation. The agreement between EPYA results and thermobalance results on the compositions of the condensed phases at 1600'~ was good considering the diffi- culties involved in achieving fast quenching, Table 1.
Fig.2
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Phase diagram for the system Mg0-Mn203-Mn0 in air. Compositions molar %.The a and c parameters of the tetragonal spinel phase were determined from the (200) and (004) reflections of three samples (TB.3, TB.4 and TB.5, Figure 2) quenched from the single phase region and the mean linear dimension a. = (a2c)
93
is plotted against composition in Figure 3. Similarly, the lattice parameter, a, of the cubic(Mg,Mn)O phase obtained from samples quenched from 1600-1650'~ in the single phase
JOURNAL DE PHYSIQUE
Fig.3 - Mean l i n e a r dimension a o ( % o f t h e (Mg,Mn)MnZ04 L t e t r a g o n a l c e l l v e r s u s MgO c o n t e n t o f t h e (Mg,Mn)Mn204 phase. ag = (a2c13
.
0 molar 8 on t h e a b s c i s s a r e p r e s e n t s Mn304.r e g i o n , i s shown p l o t t e d a g a i n s t composition i n F i g u r e 4 and i s compared w i t h t h e d a t a o f Woermann and Muan /lo/ o b t a i n e d under an oxygen p a r t i a l p r e s s u r e of 0.101 MPa a t 15C0°c. I t i s s e e n t h a t a t h i g h e r t e m p e r a t u r e s i n a i r t h e c u r v e s approach t h a t o f Woermann and Muan as t h e d i v a l e n t manganese i o n i s s t a h i l i s e d .
Fig.4
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Cubic l a t t i c e parameter a ( A ) of (Mg,Mn)O v e r s u s composition, molar 8.Table 1 - Compositions of condensed phases (molar % ) in equilibrium determined from thermobalance and EPMA.
IV.2 The svstem Ma0-Mn901-Mn0-CaSi03 in air
In the quaternary system two isoplethal sections at 2.10 molar % and 4.12 molar %
CaSiOg were examined and the results were projected onto the MgO.Mn203-Mn0 face of the tetrahedron Figure 5A and B. It should be noted that the oxygen isobaric sec- tion through the tetrahedron will be an irregular curved surface and hence project- ion onto the composition plane will distort the results. At the temperatures chosen the silicate phase was present as liquid and (Mg,Mn)O and (Mg.Mn)Mn204 solid solu- tions were the only crystalline phases present. The diagrams are therefore useful for assessing the effects of small amounts of CS phase on phase stability by compa- rison with Figure 2.
Fig.5
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Isotherms determined in air in the isoplethal section [A] Mg0-Mn20g-Mn0-2.1 molar % CaSiOg[B] Mg0-Mn203-Mn0-4.12 molar % CaSi03
Compositions are in molar %. Dash-dot lines are reaction paths.
EPM Results (b)
Sample 7 '
Thermobalance Data (a) Mn203
7.5 25.0 7.5 17.0
9.3 34.2 Temp.
O c
1650 1650 1650 1650 1550 1550 NO.
TB.3.3 TB.3.3 TB. 3 TB. 3 T B . ~ TB.4
Mn203 8.1 18.3 8.1 17.6 9.3 34.2
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Phase
(Mg,Mn)O Liquid
(Mg,Mn)O Liquid
(Mg,Mn)O (Mg,Mn)Mn204
MnO 81.7 71.3 81.7 77.2 56.8 48.3 MnO
82.2 75.7 82.2 76.5 54.6 47.0 Compn.
MgO 10.8 3.8 10.8 5.8 33.9 17.4 MgO
9.7 5.9 9.7 5.9 36.1 18.8 Mgo
5.7 5.7 9.3 9.3 17.8 17.8
Mno 94.3 94.3 90.7 90.7 82.2 82.2
JOURNAL DE PHYSIQUE
The isothermal section at 1600°c of compositions containing up to 25.8 molar% CaSiq is shown projected onto the plane Mg0-Mn0-CaSi03 Figure 6. Only two condensed phases were present in the region examined, liquid and (Mg.Mn)O solid solution and tie lines were determined from EPMA for samples 13 and 23. In plotting these results the small CaO and Si02 solubility (< 1.0 wt.%) in the (Mg.Mn)O phase were disregarded, Table 2.
The boundary line between the all liquid field and the (Mg,Mn)O
+
liquid phase field is the interesection of the isobaric surface (0.021 MPa P across the MgO-MngOg- MnO-CaSiOg tetrahedron with the MgO-MnO-CaSiOg plane. Ito2)is not a true boundary line since its position will change as the oxygen partial pressure is varied.V
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FURTHER DISCUSSIONFrom a refractory viewpoint a magnesia lining in air could, in theory, absorb up to 63 molar % MnO before liquid formation occurs. A magnesia brick contaminated with MnO is represented by compositions lying close to the MgO corner. In air, the appea- rance of a second crystalline phase, (Mg,Mn)Mn204, would occur only at temperatures below 1 3 0 0 ~ ~ and this temperature is lowered still further at lower oxygen partial pressures. When CaSi03 is added to the system a decrease in the Mn203/Mn0 ratio is found when comparing mixtures of the same Mn20g/Mn0 ratio at the same temperature, Figure 7.
The presence of liquid silicate lowers the temperature stability range of (Mg,Mn)O solid solution. In Figure 6 the slope of the determined tie lines show that the ad- dition of MnO to MgO-CaSiOg mixtures lowers the MgO content of the liquid. This
Fig.6
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The Mg0-Mn0-CaSi03 isothermal section at 1600°c in air. Compositions are in molar %, point b was taken from Phase Diagrams for Ceramists (1964) Figure 598.Dash-dot lines are tie lines from EPMA. P = (Mg,Mn)O, L = liquid.
supports the observation of Brunner /11,12/ that MnO additions to MgO bricks reduced slag attack at CaO/Si02 molar ratio = 1.0. Glasser /8/ studied the system CaO-MnO- SiOp at 10-7 MPa Po2 and reported liquid in equilibrium with solid phase at 1600°c for a composition 67 mol.% MnO, 37 mol.% CaSiOg. This would displace the boundary line in Figure 6 towards the CaSiOg corner lowering still further the MgO solubility in the liquid phase. Thus, small additions of MnO to magnesia brick compositions may have a beneficial effect in the presence of liquid slag.
Table 2
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Starting compositions and EPMA results (molar % ) on two samples quenched from 1 6 0 0 ~ ~ in air.Fig. 7
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Variation of O/Mn ratio versus CaSi03 content (molar % ) in Mg0-Mn203-Mn0- CaSi03 compositions at 1450 and 1 5 0 0 ~ ~ .REFERENCES Sample
/1/ Otto, E.M., 3. Electrochem. Soc.
112
(1965) 367./2/ Fukunaga, 0.. Takahashi, K., Fujita, T. and Yoshimoto, J., Mat.Res. Bull.
4 (1969) 315.
/3/ ~ai;n, Jr. W.C. and Muan, A., Am. 3. Science
258
(1960) 66./4/ Morris, A.E. and Muan, A., J. Metals (August 1966) 957.
/5/ Hed, A.Z. and Tannhauser, D.S., J. Electrochem. Soc.
114
(1967) 314./6/ Riboud, P.V. and Muan, A., 3. Am. Ceram. Soc.
46
(1963) 33./7/ Glasser, F.P., Am. 3. Science
259
(1961) 46./8/ Glasser, F.P., J. Am. Ceram. Soc.
5
(1962) 242./9/ Glasser, F.P., and Osborn, E.F., 3. Am. Ceram. Soc.
43
(1960) 132./lo/ Woermann, E. and Muan, A., Mat. Res. B U ~ . 5 (1970) 779.
/11/ Brunner, M., Trans. Brit. Ceram. Soc.
62
(i963) 813./12/ Brunner, M., Jerkontorets Annaler
148
(1964) 470.Phases present (Mg,Mn)O Liquid
(Mg,Mn)O Liquid No.
13
23
ACKNOWLEDGEMENT
The authors are grateful to Magnesita S.A. for the study leave granted to one of them (VAGO) which enabled the work to be carried out.
I
Compcsition Composition
MgO 82.6
17.0
MnO 10.695 12.85 70.78 67.52 MgO
89.25 19.80 27.67 10.72 MnO
13.3
77.1
Si02 0.005 33.69 0.66 11.20 CaSiOj
4.1
5.9
CaO 0.05 33.66 0.89 10.56
