Phase Equilibrium Studies in the “MnO”-Al2O3-SiO2 System

GHASEM ROGHANI, EVGUENI JAK, and PETER HAYES

Phase relations and the liquidus surface in the system “MnO”-Al2O3-SiO2 at manganese-rich alloysaturation have been investigated in the temperature range from 1373 to 1773 K. This system containsthe primary-phase fields of tridymite and cristobalite (SiO2); mullite (3Al2O3?2SiO2); corundum(Al2O3); galaxite (MnO?Al2O3); manganosite (MnO); tephroite (2MnO?SiO2); rhodonite (MnO?SiO2);spessartine (3MnO?Al2O3?SiO2); and the compound MnO?Al2O3?2SiO2.

I. INTRODUCTION temperatures: cristobalite and tridymite (SiO2); mullite(3Al2O3?2SiO2); corundum (Al2O3); rhodonite (MnO?IT has been shown[1,2] that in oxide systems, manganese SiO2); tephroite (2MnO?SiO2); galaxite (MnO?Al2O3);can exist as Mn4+, Mn3+, and Mn2+ ions; the relative propor-spessartine (3MnO?Al2O3?3SiO2); and the compoundtions of these ionic species present will vary with phase,2MnO?2Al2O3?5SiO2.temperature, and oxygen partial pressure. Under highly

In the present article, the compound having the chemicalreducing conditions, that is, at low oxygen partial pressures,formula 3MnO?Al2O3?3SiO2 will be referred to as “spessar-manganese is present predominantly in the Mn2+ form.tine” rather than “spessartite,” to conform with the nomen-The focus of the present study is to experimentally deter-clature used to describe garnet minerals.[9]

mine the phase equilibria and liquidus of the pseudoternaryThe ternary compound 3MnO?Al2O3?3SiO2 was shown[8]

system “MnO”-Al2O3-SiO2. This system is directly relevantby X-ray powder diffraction studies to have a crystal struc-to several important metallurgical processes, for example,ture resembling natural garnet and was thought to “proba-steel deoxidation and ferro-manganese and silico-manga-bly” melt congruently at 1473 K (1200 8C). The crystalnese alloy smelting. Previous studies of the phase equilibriastructure of the ternary compound 2MnO?2Al2O3?5SiO2in the binary subsystems have been critically reviewedremained unclear, however and, according to Snow,[8]

in articles which address “MnO”-SiO2,[3] “MnO”-Al2O3,[4]

“melts incongruently to form mullite and liquid; the upperand Al2O3-SiO2.[5,6]

limit of this reaction has been determined to be 1473 KLiquidus temperatures in the “MnO”-Al2O3-SiO2 system(1200 8C).”were first investigated using thermal analysis in iron cruci-

Galakhov[10] used a combination of equilibration andbles and optical metallographic techniques.[7] Melts con-quenching, and hot-stage microscope techniques to deter-taining up to 30 wt pct Al2O3 were studied, and liquidusmine the liquidus temperatures under argon gas betweentemperatures between 1348 and 1518 K (1075 8C to 12451923 and 2123 K (1650 8C to 1850 8C) in the mullite8C) were measured. For mixtures containing MnO/SiO2(3Al2O3?2SiO2) and corundum (Al2O3) primary-phaseweight ratios between approximately 0.3 and 0.7, the liq-fields.uidus temperatures were reported to decrease until Al2O3

The phase diagram of the “MnO”-Al2O3-SiO2 systemconcentrations exceeded 20 to 25 wt pct; further Al2O3under reducing conditions, shown in Figure 1,[11] is basedadditions resulted in increases in the liquidus. Two ternaryon the experimental results obtained by earlier workers.[8,10]compounds, 2MnO?Al2O3?SiO2 and MnO?Al2O3?2SiO2,

were reported and said to melt at 1448 K (1175 8C) and In Figure 1, the galaxite (MnO?Al2O3) is shown to melt1403 K (1130 8C), respectively. The crystal structures of incongruently. The assessment by Eriksson et al.,[4] basedthese “manganese gehlenite” and “manganese anorthite” on subsequent experimental information obtained underphases were not confirmed by X-ray powder diffraction. controlled oxygen partial pressures,[12] indicates, however,

The low-melting-temperature region of the “MnO”- that galaxite melts congruently at 2108 K (1835 8C) withAl2O3-SiO2 system was investigated by equilibrating syn- binary eutectics between MnO?Al2O3 and Al2O3 at 2030 Kthetic oxide mixtures at temperature in platinum and in (1757 8C) and between MnO?Al2O3 and MnO at 1803 Kiron crucibles under flowing purified nitrogen gas.[8] Oxy- (1530 8C). The presence of congruently melting galaxitegen was removed from the gas phase by passing the gas in the “MnO”-Al2O3 system represents a significant differ-over copper gauze at 773 K and an Mn-Fe-Si alloy at ence in behavior, one which is likely to affect a majorthe reaction temperature. On cooling, the specimens were portion of the ternary diagram. In addition, a review of allsubjected to metallographic examination, and selected sam- of the experimental studies undertaken to date shows thatples were studied using X-ray powder diffraction. The fol- there are no experimental data available in the manganositelowing phases have been reported to be stable at liquidus primary-phase field in this pseudoternary system. Further

careful experimental measurement is, therefore, requiredto accurately determine the liquidus, particularly in themanganese-rich corner of the phase diagram. In addition,GHASEM ROGHANI, formerly Postdoctoral Research Fellow,

EVGUENI JAK, Research Director, and PETER HAYES, Director, Associ- different approaches have been used to control the oxygenate Professor, are with PYROSEARCH, Pyrometallurgy Research Centre, partial pressure of the system, and these conditions are notSchool of Engineering, The University of Queensland, Brisbane, Queens-

well defined. Finally, compositions of solid phases haveland, 4072, Australia. Contact e-mail: E.Jak@minmet.ug.edu.auManuscript submitted May 21, 2002. not been measured by previous researchers. The aim of the

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 33B, DECEMBER 2002—827

Fig. 1—Liquidus of the “MnO”-Al2O3-SiO2 system.[11]

present study is to overcome these uncertainties and to In the equilibration experiments, the slag and alloy sam-provide new data on this metallurgically important system. ples, contained, again, in molybdenum envelopes, were then

raised slowly until they reached the hot zone and kept closeto the thermocouple. The samples were heated for timesII. EXPERIMENTALranging from 0.5 to 3 hours at temperatures approximately

The general experimental procedures used in the present 50 K above the desired equilibrium temperature and wereinvestigation have been described in previous articles.[13,14,15] then cooled to the desired temperature and equilibrated forThe present investigation was carried out with a Mn-rich times from 1 to 72 hours. After local equilibrium betweenalloy. Synthetic slag was prepared from oxide powders of the solid and liquid phases at the desired temperatures hadSiO2 (99.9 wt pct purity), Al2O3 (99.8 wt pct purity, supplied been achieved, the sample was quenched rapidly to roomby Aldrich), and MnO (99.5 wt pct purity, supplied by ALFA temperature by dropping it directly from the hot zone intoChemicals). Manganese powder (99.9 wt pct purity, supplied the quenching water, which was placed beneath the reactionby Aldrich) and silicon (99.999 wt pct purity, supplied by tube. The lower end of the furnace tube was sealed with aBalzers) were used as starting materials for the manganese thin plastic film, which allowed the hot sample to passalloy. The silico-manganese alloys were prepared by melting through on its release from the hot zone of the furnace. Thethe metal powders in a carbon crucible at 1573 K (1300 8C). samples were mounted and polished for optical metallo-

Various synthetic master slags were prepared by mixing graphic examination and microanalysis.the powders in the desired proportions, pelletizing them, and The phases present in quenched slag samples were firstplacing the samples in the molybdenum foil envelopes. The examined using the optical microscope. The phases presentsamples were suspended by molybdenum wire and heated were then identified by electron probe X-ray microanalysisin a vertical LaCrO3 resistance furnace in a gas-tight recrys- (EPMA) using a JEOL* 8800L instrument with wavelength-tallized alumina reaction tube. After treatment and coolingin ultrahigh-purity nitrogen, the samples were crushed and *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.pelletized and, again, heated at 1773 K (1500 8C) for 72

dispersive spectrometers. A rhodonite (MnSiO3) standardhours, to ensure homogeneity of the particles. The tempera-was used for manganese, a spessartine (3MnO?Al2O3?3SiO2)ture was measured by the working Pt/Pt13 pct Rh thermocou-standard was used for aluminum, and a wollastoniteple positioned next to the sample. This working(CaSiO3) standard was used for silicon calibration. Thesethermocouple was periodically tested against a calibratedstandards were obtained from Charles M. Taylor Company.standard thermocouple. The overall temperature accuracy

was estimated to be 65 K. The Duncumb–Philibert ZAF correction procedure supplied

828—VOLUME 33B, DECEMBER 2002 METALLURGICAL AND MATERIALS TRANSACTIONS B

Table I. Experimental Data for the “MnO”-Al2O3-SiO2with the JEOL 8800L instrument was applied. The averageSystem in Equilibrium with Mn/Si Alloyaccuracy of the EPMA measurements was within 61 wt

pct. The EPMA measurements revealed that molybdenum (Wt Pct)T Phases in Phasemetal and oxide concentrations in the Mn-rich alloy and(K) Equilibrium Name “MnO” SiO2 Al2O3oxide phases, respectively, were negligible.

Equilibria (Liquid 1 Corundum C 1 Mn/Si Alloy)1393 L 1 C L 33.8 44.9 21.3

III. RESULTS C 0.9 0.1 99.01423 L 1 C L 37.6 40.4 22.0Details of the compositions of the liquid and the solid

C 1.0 0.1 98.9phases determined in these studies are given in Table I. The1473 L 1 C L 34.3 42.7 23.1experimental results for the liquidus of the ternary system

C 0.8 0.1 99.2“MnO”-Al2O3-SiO2 are presented in Figures 2(a) and (b). 1473 L 1 C L 41.4 36.2 22.3The liquidus surfaces for the binary systems MnO-SiO2, C 1.0 0.0 99.0Al2O3-SiO2, and MnO-Al2O3 are taken from optimized 1473 L 1 C L 27.9 50.3 21.8F*A*C*T model predictions of these systems.[3,4,5]

C 0.7 0.1 99.2This “MnO”-Al2O3-SiO2 system shown in Figures 2(a) 1473 L 1 C L 27.8 50.2 22.0

C 0.8 0.2 99.1and (b) contains nine primary-phase fields: tridymite and1523 L 1 C L 30.9 46.2 23.0cristobalite (SiO2, or S); mullite (3Al2O3?2SiO2, or A3S2);

C 0.9 0.1 99.0corundum (Al2O3, or A); galaxite (MnO?Al2O3, or MA);1523 L 1 C L 34.8 41.5 23.7manganosite (MnO, or M); tephroite (2MnO?SiO2, or M2S);

C 0.9 0.3 98.8rhodonite (MnO?SiO2 or MS); spessartine (3MnO?Al2O3? 1523 L 1 C L 40.5 35.5 24.03SiO2, or M3AS3); and MnO?Al2O3?2SiO2, (MAS2). The C 1.0 0.1 99.0liquidus isotherms in this system were constructed between 1573 L 1 C L 32.1 43.3 24.61373 and 1773 K (1100 8C to 1500 8C) at 50 K intervals. C 0.7 0.0 99.3

Examples of typical microstructures in the system of 1573 L 1 C L 39.5 35.5 25.0“MnO”-Al2O3-SiO2 observed using backscattered electron C 0.9 0.0 99.1

1573 L 1 C L 36.7 38.7 24.7imaging are presented in Figures 3 through 9. Figure 3C 0.9 0.0 99.1illustrates the microstructure of the tridymite (SiO2) crystal-

1573 L 1 C L 35.0 40.0 24.9line phase in equilibrium with the liquid oxide and Mn-SiC 0.9 0.1 99.1alloy. The platelike microstructure of mullite (3Al2O3?2SiO2)

1573 L 1 C L 32.5 41.7 25.8is illustrated in Figure 4. Figure 5 shows the angular corun-C 0.8 0.0 99.1dum (Al2O3) crystals. The coexistence of corundum and 1573 L 1 C L 27.9 46.8 25.3

galaxite spinel (MnO?Al2O3) can be seen in Figure 6. Figure C 0.8 0.1 99.27 shows the microstructure of a sample containing rounded 1623 L 1 C L 33.1 39.4 27.5manganosite (MnO) phase. The tephroite phase (2MnO? C 0.9 0.1 99.0SiO2) is shown in Figure 8, and the coexistence of tephroite 1623 L 1 C L 36.8 35.8 27.4

C 0.8 0.0 99.2and rhodonite (MnO?SiO2) is shown in Figure 9.1623 L 1 C L 28.4 44.9 26.7While earlier researchers have used differential thermal

C 0.9 0.1 99.0analysis, optical metallography, and X-ray powder diffrac-1623 L 1 C L 39.4 33.3 27.3tion techniques to characterize phase equilibria and to iden-

C 0.9 0.0 99.1tify the presence of particular phases in the system, those1673 L 1 C L 35.8 35.0 29.3approaches do not provide direct measurement of the chemi- C 0.8 0.0 99.2

cal compositions of the individual phases. In contrast to 1673 L 1 C L 39.1 31.9 29.1these earlier studies, in the present investigation, EPMA of C 0.8 0.0 99.2all phases has been carried out. Data on the compositions 1673 L 1 C L 27.5 43.6 28.9of the compounds are summarized in Table II. Analysis of C 0.8 0.1 99.1

1723 L 1 C L 36.7 31.0 32.4the EPMA data obtained in the present studies shows thatC 0.7 0.0 99.3the standard deviation of all of the analyses carried out is

1773 L 1 C L 29.9 36.1 34.1less than 0.5 pct. It is, therefore, assumed that compoundsC 0.5 0.0 99.5containing maximum concentrations of less than 1 wt pct

1773 L 1 C L 36.8 25.6 37.6of a particular component do not exhibit a solid solutionC 0.6 0.0 99.4involving that component, since these concentrations are

less than the limits of uncertainty of the measurement. For Equilibria (Liquid 1 Mullite 1 Mn/Si Alloy)1573 L 1 A3S2 L 22.7 54.7 22.7example, the following phases contain little or no MnO:

A3S2 0.7 27.9 71.5tridymite and cristobalite (SiO2, or S); mullite (3Al2O3?1623 L 1 A3S2 L 24.7 50.2 25.12SiO2, or A3S2); and corundum (Al2O3, or C). The tephroite

A3S2 0.7 26.9 72.5(2MnO?SiO2, or M2S) and rhodonite (MnO?SiO2, or MS)1623 L 1 A3S2 L 24.5 51.0 24.6phases contain little or no Al2O3. The galaxite (MnO?Al2O3, A3S2 0.7 26.8 72.5

or MA) and manganosite (MnO, or M) phases contain little or 1623 L 1 A3S2 L 20.7 56.5 22.8no SiO2. The ternary compound MnO?Al2O3?2SiO2, (MAS2) A3S2 0.7 27.0 72.3can be regarded as stoichiometric within the limits of uncer- 1673 L 1 A3S2 L 24.0 49.3 26.7tainty specified previously. According to Deer et al.,[9] the A3S2 1.0 26.8 72.2ternary compound spessartine (3MnO?Al2O3?3SiO2 or

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 33B, DECEMBER 2002—829

Table I. Experimental Data for the “MnO”-Al2O3-SiO2Table I. Experimental Data for the “MnO”-Al2O3-SiO2

System in Equilibrium with Mn/Si Alloy—Continued System in Equilibrium with Mn/Si Alloy-Contiued

(Wt Pct) (Wt Pct)T Phases in PhaseT Phases in Phase(K) Equilibrium Name “MnO” SiO2 Al2O3 (K) Equilibrium Name “MnO” SiO2 Al2O3

Equilibria (Liquid 1 Mullite A3S 1 Mn/Si Alloy)—Continued Equilibria (Liquid 1 Galaxite MA 1 Mn/Si Alloy)—Continued1573 L 1 MA L 52.2 30.0 17.81673 L 1 A3S2 L 21.4 53.6 25.1

A3S2 0.6 27.1 72.4 MA 40.4 0.3 59.41623 L 1 MA L 62.7 22.4 14.91673 L 1 A3S2 L 19.0 57.1 23.8

A3S2 0.9 27.6 71.5 MA 40.1 0.0 59.91623 L 1 MA L 57.4 26.0 16.71673 L 1 A3S2 L 17.4 59.2 23.5

A3S2 0.8 26.9 72.4 MA 39.7 0.1 60.31623 L 1 MA L 52.3 28.3 19.41673 L 1 A3S2 L 16.6 61.5 21.9

A3S2 0.7 28.0 71.3 MA 40.4 0.1 59.51623 L 1 MA L 48.2 29.6 22.21673 L 1 A3S2 L 15.4 62.8 21.8

A3S2 0.6 26.8 72.6 MA 40.7 0.1 59.21673 L 1 MA L 61.9 19.4 18.71723 L 1 A3S2 L 18.4 55.6 26.0

A3S2 0.6 27.5 71.9 MA 40.4 0.0 59.61673 L 1 MA L 49.6 27.3 23.11723 L 1 A3S2 L 16.6 59.2 24.2

A3S2 0.8 26.8 72.4 MA 40.0 0.1 59.91723 L 1 MA L 66.8 12.9 20.41723 L 1 A3S2 L 16.2 59.4 24.4

A3S2 0.5 28.1 71.4 MA 41.0 0.1 59.01723 L 1 MA L 59.6 19.2 21.21723 L 1 A3S2 L 14.8 63.1 22.2

A3S2 0.8 27.8 71.4 MA 40.4 0.0 59.61723 L 1 MA L 49.0 23.8 27.21773 L 1 A3S2 L 20.0 49.0 31.0

A3S2 0.5 27.8 71.7 MA 40.3 0.0 59.71773 L 1 A3S2 L 9.6 71.4 19.1

Equilibria (Liquid 1 Rhodonite MS 1 Mn/Si Alloy)A3S2 0.3 26.7 73.01373 L 1 MS L 42.2 40.7 17.1

MS 52.7 44.7 2.6Equilibria (Liquid 1 Manganosite M 1 Mn/Si Alloy)1573 L 1 M L 66.0 23.1 10.9 1393 L 1 MS L 42.6 42.0 15.4

MS 53.7 45.3 1.0M 99.7 0.0 0.31573 L 1 M L 67.8 26.6 5.6 1473 L 1 MS L 47.9 42.0 10.1

MS 53.2 45.7 1.1M 99.6 0.0 0.41623 L 1 M L 69.2 24.9 6.0

Equilibria (Liquid 1 Silica S 1 Mn/Si Alloy)M 99.9 0.0 0.11423 L 1 S L 31.1 49.3 19.61673 L 1 M L 69.4 22.5 8.1

S 1.5 97.8 0.7M 99.8 0.0 0.21473 L 1 S L 31.7 50.4 17.91723 L 1 M L 69.9 21.2 9.0

S 1.0 98.8 0.3M 99.8 0.0 0.21473 L 1 S L 27.1 53.2 19.7

S 1.1 98.4 0.5Equilibria (Liquid 1 Tephroite M2S 1 Mn/Si Alloy)1473 L 1 M2S L 52.5 37.6 9.9 1523 L 1 S L 47.5 47.1 5.4

S 0.7 99.3 0.0M2S 69.6 30.0 0.41573 L 1 M2S L 66.4 30.4 3.2 1523 L 1 S L 39.4 49.1 11.5

S 0.8 99.1 0.1M2S 69.6 30.3 0.11573 L 1 M2S L 62.9 36.6 0.5 1573 L 1 S L 47.3 47.9 4.9

S 1.1 98.8 0.1M2S 69.4 30.6 0.01573 L 1 S L 43.2 49.4 7.5

Equilibria (Liquid 1 Galaxite MA 1 Mn/Si Alloy) S 0.9 99.0 0.11473 L 1 MA L 46.4 34.8 18.8 1573 L 1 S L 37.2 51.4 11.4

MA 41.3 0.1 58.6 S 1.1 98.7 0.21473 L 1 MA L 46.1 36.2 17.7 1573 L 1 S L 27.6 55.3 17.1

MA 40.3 0.1 59.7 S 1.8 97.3 0.91523 L 1 MA L 59.3 28.9 11.8 1623 L 1 S L 53.1 46.9 0.0

MA 41.1 0.1 58.8 S 1.1 98.9 0.01523 L 1 MA L 55.6 30.1 14.4 1623 L 1 S L 49.5 48.2 2.2

MA 40.3 0.2 59.5 S 0.8 99.1 0.01523 L 1 MA L 43.8 34.2 22.0 1623 L 1 S L 39.6 51.6 8.8

MA 41.3 0.1 58.6 S 1.0 98.7 0.31573 L 1 MA L 62.0 25.3 12.7 1623 L 1 S L 31.2 54.5 14.3

MA 41.2 0.1 58.6 S 1.2 98.3 0.51573 L 1 MA L 58.3 27.7 14.0 1673 L 1 S L 51.4 48.6 0.0

MA 41.4 0.0 58.5 S 0.7 99.3 0.01573 L 1 MA L 56.1 28.6 15.4 1673 L 1 S L 37.3 54.2 8.6

MA 40.5 0.1 59.4 S 0.6 99.1 0.31573 L 1 MA L 55.7 29.1 15.2 1673 L 1 S L 29.2 57.5 13.3

MA 41.2 0.1 58.7 S 0.9 98.8 0.3

830—VOLUME 33B, DECEMBER 2002 METALLURGICAL AND MATERIALS TRANSACTIONS B

Table I. Experimental Data for the “MnO”-Al2O3-SiO2Table I. Experimental Data for the “MnO”-Al2O3-SiO2

System in Equilibrium with Mn/Si Alloy—Continued System in Equilibrium with Mn/Si Alloy—Continued

(Wt Pct) (Wt Pct)T Phases in PhaseT Phases in Phase(K) Equilibrium Name “MnO” SiO2 Al2O3 (K) Equilibrium Name “MnO” SiO2 Al2O3

Equilibria (Liquid 1 Silica S 1 Mn/Si Alloy)—Continued Equilibria (Liquid 1 Silica S 1 MAS2 1 Mn/SiAlloy)—Continued1673 L 1 S L 18.4 63.1 18.6

S 0.9 98.2 0.9 1393 L 1 MAS2 L 30.8 48.6 20.61 S1723 L 1 S L 50.6 49.4 0.0

S 0.6 99.4 0.0 MAS2 23.6 42.5 34.0S 1.0 98.5 0.51723 L 1 S L 38.0 54.5 7.5

S 0.7 99.2 0.1Equilibria (Liquid 1 Tephroite M2S 1 Manganosite M 1 Mn/Si1723 L 1 S L 24.7 62.9 12.4

Alloy)S 0.7 99.1 0.21573 L 1 M2S L 67.7 27.3 5.01773 L 1 S L 12.4 72.4 15.2

1 MS 0.3 99.6 0.1M2S 69.7 30.2 0.1

M 99.6 0.1 0.3Equilibria (Liquid 1 Mullite A3S2 1 Corundum C 1 Mn/SiAlloy)

Equilibria (Liquid 1 Tephroite M2S 1 Galaxite MA 1 Mn/Si1573 L 1 A3S2 L 26.2 48.8 25.0Alloy)1 C

1473 L 1 M2S L 56.3 31.7 11.9A3S2 0.7 27.0 72.21 MAC 0.8 0.1 99.2

M2S 69.6 30.3 0.21723 L 1 A3S2 L 25.7 42.7 31.7MA 41.8 0.1 58.11 C

A3S2 0.8 26.5 72.8 Equilibria (Liquid 1 Tephroite M2S 1 Rhodonite MS 1 Mn/SiC 0.7 0.0 99.3 Alloy)

1393 L 1 M2S L 46.7 38.3 15.0Equilibria (Liquid 1 Mullite A3S2 1 Silica S 1 Mn/Si1 MSAlloy)

M2S 69.8 30.2 0.01473 L 1 A3S2 L 25.3 53.8 20.9MS 53.1 45.7 1.21 S

A3S2 0.8 28.0 71.2 Equilibria (Liquid 1 Galaxite MA 1 Corundum C 1 Mn/Si Alloy)S 0.7 98.7 0.6 1573 L 1 MA 1 L 40.8 34.8 24.4

1523 L 1 A3S2 L 23.2 55.0 21.8 C1 S MA 40.0 0.1 59.9

A3S2 1.0 28.5 70.5 C 1.0 0.2 98.9S 0.8 98.7 0.5 1623 L 1 MA 1 L 41.8 30.9 27.2

1573 L 1 A3S2 L 21.0 57.7 21.3 C1 S MA 40.6 0.2 59.2

A3S2 1.0 28.5 70.5 C 1.0 0.2 98.9S 0.7 99.0 0.3 1673 L 1 MA 1 L 40.9 29.0 30.1

1623 L 1 A3S2 L 18.2 60.3 21.5 C1 S MA 39.8 0.0 60.2

A3S2 0.6 27.4 72.1 C 0.7 0.0 99.3S 0.7 99.1 0.3

Equilibria (Liquid 1 Galaxite MA 1 Manganosite M 1 Mn/Si1673 L 1 A3S2 L 14.0 66.1 19.9Alloy)1 S

1523 L 1 MA 1 L 62.6 26.4 11.0A3S2 0.5 28.1 71.5MS 0.3 99.4 0.2

MA 40.2 0.2 59.71723 L 1 A3S2 L 11.7 69.4 19.0M 99.6 0.0 0.41 S

1573 L 1 MA 1 L 64.5 23.2 12.3A3S2 0.5 28.3 71.2MS 0.4 99.1 0.5

MA 41.3 0.1 58.71773 L 1 A3S2 L 7.7 76.0 16.3M 99.4 0.0 0.51 S

1623 L 1 MA 1 L 65.3 19.9 14.7A3S2 0.5 28.7 70.9MS 0.3 99.7 0.0

MA 42.0 0.2 57.8Equilibria (Liquid 1 Silica S 1 Corundum C 1 Mn/Si Alloy) M 99.7 0.0 0.31423 L 1 C 1 S L 29.3 49.1 21.6 1673 L 1 MA 1 L 66.4 16.9 16.8

C 0.6 0.1 99.3 MS 1.4 97.6 1.0 MA 40.9 0.0 59.0

M 99.7 0.0 0.3Equilibria (Liquid 1 Silica S 1 MAS2 1 Mn/Si Alloy)1373 L 1 MAS2 L 33.6 47.0 19.5 Equilibria (Liquid 1 Rhodonite MS 1 MAS2 1 Mn/Si Alloy)

1 S 1333 L 1 MS 1 L 38.0 43.9 18.1MAS2 24.6 41.3 34.1 MAS2

S 1.4 97.7 0.9 MS 51.8 45.0 3.2MAS2 23.9 41.8 34.3

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 33B, DECEMBER 2002—831

Table I. Experimental Data for the “MnO”-Al2O3-SiO2 measured following equilibration. Uncertainties regardingSystem in Equilibrium with Mn/Si Alloy—Continued changes in slag composition during equilibration have, there-

fore, been eliminated. The present studies have also been(Wt Pct)T Phases in Phase carried out so as to ensure that there is local equilibrium(K) Equilibrium Name “MnO” SiO2 Al2O3 between the condensed phases, in particular, between the

Equilibria (Liquid 1 Rhodonite MS 1 Silica S 1 Mn/Si Alloy) manganese - silicon alloy and the slag phase. The effective1373 L 1 MS 1 S L 35.8 46.6 17.6 (but unknown, not measured) oxygen partial pressure in the

MS 52.4 45.5 2.1 slag phase is that in equilibrium with the metal phase, therebyS 0.7 96.4 2.9 minimizing the concentration of Mn3+ in the slag. In addition,

1423 L 1 MS 1 S L 38.7 47.8 13.6 the present technique of controlling the equilibration temper-MS 53.6 45.7 0.7 ature and varying the slag composition enables the liquidusS 1.4 98.1 0.5

isotherms to be determined with far greater accuracy than1473 L 1 MS 1 S L 42.9 47.7 9.5previously possible.MS 53.5 46.2 0.3

S 1.1 98.8 0.1

A. Comparison with Previous Studies

The liquidus surface and the primary-phase fields deter-M3AS3) is an end member of a number of naturally occurringisomorphous garnets in which M2+ and M3+ ions can be mined in the present study (Figure 2) differ in a number of

significant respects from those reported by Snow[8] andreplaced by cations of similar charge and atomic radius.Muan and Osborn.[11] Refer, for example, to the dotted linesin Figure 1, which show the 1573 K (1300 8C) liquidus

IV. DISCUSSION isotherm as determined in the present investigation.The galaxite primary-phase field as now reported extendsThe experimental techniques used in the present investiga-

tion differ in several respects from those used by previous from the eutectic between MnO?Al2O3 and Al2O3 at highAl2O3 concentrations to the eutectic between MnO?Al2O3investigators. The present study represents the first investiga-

tion carried out on this ternary system in which the composi- and MnO. As a result, the galaxite (MA), tephroite (M2S),and rhodonite (MS) primary-phase fields are much largertions of the liquid and solid phases formed have been directly

(a)

Fig. 2—(a) Liquidus of the “MnO”-Al2O3-SiO2 system in equilibrium with Mn-Si alloy showing the data obtained in the present study.

832—VOLUME 33B, DECEMBER 2002 METALLURGICAL AND MATERIALS TRANSACTIONS B

(b)

Fig. 2—(b) Liquidus of the “MnO”-Al2O3-SiO2 system in equilibrium with Mn-Si alloy.

than previously reported,[11] and the manganosite (M), spes- itself contains 36 wt pct SiO2. The results of the presentinvestigation show (Table I) that the galaxite primary-phasesartine (M3AS3), and corundum (A) primary-phase fields

are smaller in size. The position of the silica primary-phase is stable below 36 wt pct SiO2. Snow[8] goes on to state that“spessartine probably melts congruently, but its slow meltingfield determined in the present study is in good agreement

with that given in Figure 1. The position of the corundum/ and the fact the liquid loses manganese on heating makesthis point difficult to determine.” In his discussion of themullite phase boundary agrees well with the data by Gala-

khov;[10] these data are shown in Figure 2(a). equilibrium data, he says that “because of the flatness ofthe field of primary crystallization, the exact compositionThe ternary compound 2MnO?Al2O3?SiO2, reported by

Glaser,[7] has not been observed at the liquidus, however the of this phase (spessartine) is difficult to determine, eventhough it probably has a congruent melting temperature ofcompound MnO?Al2O3?2SiO2 is found to be stable over a

range of low-melting-temperature compositions. 1200 8C 1/25 8C.” Given that Snow was not able to crosscheck the compositions of the phases after the experiments,The primary-phase fields of the compounds 3MnO?Al2O3?

3SiO2 (spessartine) and 2MnO?Al2O3?5SiO2, reported by the data obtained in the present investigation are preferredin the construction of the phase diagram. The spessartineSnow[8] to form in the low-melting-temperature region of

the system in the compositions investigated in the present phase is, therefore, shown in Figure 2 to melt incongruently,with a small, flat primary-phase field extending toward sil-study, have not been observed to form directly from the

melt. While Snow[8] clearly identified spessartine crystals ica-rich compositions.The extent of the primary-phase field of MnO?Al2O3?in his study, close inspection of the liquidus data he reports

shows that the three compositions in which spessartine is the 2SiO2 appears to be close to that attributed previously[8] to2MnO?2Al2O3?5SiO2. Snow[8] makes several points that areprimary-phase contain 36 wt pct SiO2 or greater. Spessartine

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 33B, DECEMBER 2002—833

Fig. 3—Backscattered electron image of the tridymite (SiO2) crystalline phase in equilibrium with the liquid oxide and Mn-Si alloy. Legend: A 5 alloy,S 5 silica, G 5 glass.

Fig. 4—Backscattered electron image of plate-like mullite (3Al2O3?2SiO2) in equilibrium with the liquid oxide and Mn-Si alloy. Legend: A 5 alloy, Mu 5mullite, G 5 glass.

relevant to the interpretation of the data. He states in his has been ascribed to MAS2. There clearly remains, however,some uncertainty as to the phase boundaries and liquidusconclusion that he “identified a manganese mineral having

a composition approximately 2MnO?2Al2O3?5SiO2.” He also surfaces in this relatively small region of the diagram.Snow[8] reports 13 compositions for which the liquidusstates that “M2A2S5 is very difficult to prepare, probably

because it melts incongruently to form mullite and liquid was determined, five of these being in the silica, three in thecorundum, three in the spessartine, and one in the tephroiteand has a very small field of stability on the liquidus surface”.

Snow only reports one liquidus point in the M2A2S5 primary- primary-phase fields. Comparison of these points with theproposed diagram (Figure 2) shows good agreement withinphase field, between 1399 and 1413 K (1126 8C and 1140

8C). Given that the benefit of EPMA analysis is available the experimental uncertainties reported by Snow and thepresent studies. Where small differences do occur, these areto the present investigators, 2MnO?2Al2O3?5SiO2 (M2A2S5)

834—VOLUME 33B, DECEMBER 2002 METALLURGICAL AND MATERIALS TRANSACTIONS B

Fig. 5—Backscattered electron image of angular corundum (Al2O3) crystals in equilibrium with the liquid oxide and Mn-Si alloy. Legend: A 5 alloy, C 5corundum, G 5 glass.

Fig. 6—Backscattered electron image showing the coexistence of corundum (Al2O3) and galaxite spinel (MnO?Al2O3) in equilibrium with the liquid oxideand Mn-Si alloy. Legend: A 5 alloy, C 5 corundum, MA 5 spinel, G 5 glass.

associated with steep liquidus surfaces in the silica- and demonstrate that an increasing Al2O3 concentration alsoleads to significant decreases in the liquidus in this system.corundum-phase fields.Al2O3 has the effect, in this case, of preferentially stabilizingthe liquid phase relative to pure silica and the stoichiometric,

B. Pseudobinary Sections alumina-free manganese silicates.The liquidus surfaces as a function of the Al2O3 concentra-

tion for MnO/SiO2 ratios of 0.5, 1.0, and 2.0 are shown inC. Practical ImplicationsFigure 10. While it is known from the “MnO”-SiO2 binary

that the liquidus decreases markedly with increasing MnO/ The present study has shown that the phase diagram forthe “MnO”-Al2O3-SiO2 system (Figure 2) is significantlySiO2 ratio, the sections through the ternary section clearly

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 33B, DECEMBER 2002—835

Fig. 7—Backscattered electron image of rounded manganosite (MnO) phase in equilibrium with the liquid oxide and Mn-Si alloy. Legend: A 5 alloy,MnO 5 manganosite, G 5 glass.

Fig. 8—Backscattered electron image of the tephroite phase (2MnO?SiO2) in equilibrium with the liquid oxide and Mn-Si alloy. Legend: A 5 alloy, T 5tephroite, G 5 glass.

different from that previously reported (Figure 1). These At steelmaking and casting temperatures (1775 K andabove), however, the saturation limit of Al2O3 in the liquidfindings have implications for the steelmaking and refining

practices. For example, in aluminum-free steels, the MnO/ oxide melt is almost independent of MnO/SiO2 ratio, despitethe fact that the liquidus crosses three different primary-SiO2 ratio in the steel deoxidation product should be main-

tained at greater than approximately unity to produce mallea- phase fields (mullite (A3S2), corundum (A), and galaxite(MA)) with increasing MnO/SiO2 ratio.ble liquid inclusions at temperature. If the steels also contain

aluminum, then liquid inclusions can be formed at muchV. CONCLUSIONSlower MnO/SiO2 ratios. For example, at MnO/SiO2 5 0.5,

the inclusions can contain up to 35 wt pct Al2O3 and still The phase relations, liquidus surface, as well as primary-phase fields, in the system “MnO”-Al2O3-SiO2 have, for thebe fully liquid at steelmaking and processing temperatures.

836—VOLUME 33B, DECEMBER 2002 METALLURGICAL AND MATERIALS TRANSACTIONS B

Fig. 9—Backscattered electron image showing the coexistence of tephroite (2MnO?SiO2) and rhodonite (MnO?SiO2) in equilibrium with the liquid oxideand Mn-Si alloy. Legend: A 5 alloy, R 5 rhodonite, T 5 tephroite, G 5 glass.

Table II. Summary of the Compositions of the Solid Phases Experimentally Determined in the “MnO”-Al2O3-SiO2 Systemin Equilibrium with Mn/Si Alloy

(Wt Pct) (Mol Pct)

Phase Name and Conditions “MnO” SiO2 Al2O3 “MnO” SiO2 Al2O3

Tridymite SiO2 avg 0.9 98.7 0.4 0.8 99.0 0.2Number of points 34 std 0.3 0.7 0.5 0.3 0.5 0.3Tmax 1373 K min 0.3 96.4 0.0 0.3 97.7 0.0Tmin 1723 K max 1.8 99.7 2.9 1.5 99.7 1.7

Mullite 3Al2O3?2SiO2 avg 0.7 27.5 71.8 0.8 39.0 60.1Number of points 25 std 0.2 0.7 0.7 0.2 0.8 0.8Tmax 1473 K min 0.3 26.5 70.5 0.4 37.8 58.6Tmin 1773 K max 1.0 28.7 73.0 1.2 40.5 61.5

Rhodonite MnO?SiO2 avg 53.0 45.5 1.5 49.2 49.8 1.0Number of points 8 std 0.6 0.4 0.9 0.4 0.3 0.6Tmax 1333 K min 51.8 44.7 0.3 48.3 49.2 0.2Tmin 1473 K max 53.7 46.2 3.2 49.8 50.4 2.1

Tephroite 2MnO?SiO2 avg 69.6 30.3 0.1 66.0 33.9 0.1Number of points 6 std 0.1 0.2 0.1 0.1 0.2 0.1Tmax 1393 K min 69.4 30.0 0.0 65.8 33.6 0.0Tmin 1573 K max 69.8 30.6 0.4 66.2 34.2 0.3

MnO?Al2O3?2SiO2 avg 24.0 41.9 34.1 24.7 50.8 24.4Number of points 3 std 0.4 0.5 0.1 0.5 0.5 0.1Tmax 1333 K min 23.6 41.3 34.0 24.2 50.2 24.3Tmin 1393 K max 24.6 42.5 34.3 25.3 51.5 24.6

Manganosite MnO avg 99.7 0.0 0.3 99.8 0.0 0.2Number of points 10 std 0.1 0.0 0.1 0.1 0.0 0.1Tmax 1523 K min 99.4 0.0 0.1 99.7 0.0 0.1Tmin 1723 K max 99.9 0.1 0.5 99.9 0.1 0.3

Corundum Al2O3 avg 0.8 0.1 99.1 1.2 0.1 98.7Number of points 31 std 0.1 0.1 0.2 0.2 0.1 0.3Tmax 1393 K min 0.5 0.0 98.8 0.7 0.0 98.2Tmin 1773 K max 1.0 0.3 99.5 1.4 0.5 99.3

Galaxite MnO?Al2O3 avg 40.7 0.1 59.2 49.6 0.1 50.3Number of points 27 std 0.6 0.1 0.6 0.6 0.1 0.6Tmax 1473 K min 39.7 0.0 57.8 48.6 0.0 48.8Tmin 1723 K max 42.0 0.3 60.3 50.9 0.4 51.3

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 33B, DECEMBER 2002—837

Fig. 10—The liquidus in the “MnO”-Al2O3-SiO2 system as a function of the Al2O3 concentration for MnO/SiO2 ratios of 0.5, 1.0, and 2.0 in equilibriumwith Mn-Si alloy.

first time, been determined experimentally at manganese- Queensland, for providing support for the electron probe X-ray microanalysis measurements.silicon alloy saturation in the temperature range from 1373

to 1773 K (1100 8C to 1500 8C). The compositions of thecompounds present in the ternary section have been meas- REFERENCESured using EPMA. This system contains the primary-phase

1. M. Wang and B. Sundman: Metall. Trans. B, 1992, vol. 23B, pp.fields of tridymite and cristobalite (SiO2); mullite (3Al2O3? 821-31.2SiO2); corundum (Al2O3); galaxite (MnO?Al2O3); manga- 2. S. Fritsch and A. Navrotsky: J. Am. Ceram. Soc., 1996, vol. 79, pp.

761-68.nosite (MnO); tephroite (2MnO?SiO2), rhodonite (MnO?3. G. Eriksson, P. Wu, and A.D. Pelton: Can. Metall. Q., 1994, vol. 33,SiO2), spessartine (3MnO?Al2O3?3SiO2), and the compound

pp. 13-21.MnO?Al2O3?2SiO2. 4. G. Eriksson, P. Wu, and A.D. Pelton: Calphad, 1993, vol. 17, pp.Although good agreement has been found in the liquidus 189-205.

temperature of the tridymite primary field between the 5. G. Eriksson and A.D. Pelton: Metall. Trans. B, 1993, vol. 24B, pp.807-15.results of the present study and previous work, significant

6. M. Hillert and S. Jonsson: Calphad, 1992, vol. 16 (2), pp. 193-98.changes have been made to the primary fields of corundum,7. O. Glaser: Centrblatt Mineral., 1926, Abt. A, pp. 81-96.galaxite and manganosite, tephroite, rhodonite, and spessar- 8. R.B. Snow: J. Ceram. Soc., 1943, vol. 26, pp. 11-20.

tine. The primary-phase field of MnO?Al2O3?2SiO2 has been 9. W.A. Deer, R.A. Howie, and J. Zussman: An Introduction to RockForming Minerals, 2nd ed., Longman Scientific, Harlow, UK and Johnfound to be present in the low-melting-temperature regionWiley & Sons, New York, 1992.of the system.

10. F.Ya. Galakhov: Izv. Akad. Nauk SSSR, Otdel. Khim. Nauk, 1957, pp.525-31.

11. E.F. Osborn and A. Muan: Phase Equilibria among Oxides in Steelmak-ACKNOWLEDGMENTS ing, Addison-Wesley Publishing Company, Inc., New York, NY, 1965,

Fig 109; reproduced in Slag Atlas, Verlag Stahleisen, Dusseldorf, 1995,The authors thank BHP Billiton Temco, Georgetown Plant Fig. 3.194.

(Tasmania, Australia), for providing financial support for 12. K.T. Jacob: Can. Met. Q., 1981, vol. 20, pp. 89-92.13. E. Jak, P.C. Hayes, and H.G. Lee: Met. Mater. (Korea), 1995, vol. 1this project, in particular Dr. Samir Ganguly, Technical

(1), pp. 1-8.Superintendent, who helped to initiate the study. Thanks14. B. Zhao, E. Jak, and P.C. Hayes: Metall. Trans. B, 1999, vol. 30B,also to Dr. Baojun Zhao for providing the backscattered pp. 597-605.

electron images of the crystal structures, and the staff of 15. E. Jak, B. Zhao, and P.C. Hayes: Metall. Trans. B, 2000, vol. 31B,pp. 1195-1201.the Centre for Microanalysis and Microscopy, University of

838—VOLUME 33B, DECEMBER 2002 METALLURGICAL AND MATERIALS TRANSACTIONS B