Phase-Equilibrium Data and Liquidus for the System MnO -CaO-Al 2 O 3 -SiO 2 at Al 2 O 3 /SiO 2 of 0.55 and 0.65

Transcription

1 Phase-Equilibrium Data and Liquidus for the System MnO -CaO-Al 2 O 3 -SiO 2 at Al 2 O 3 /SiO 2 of 0.55 and 0.65 GHASEM ROGHANI, EVGUENI JAK, and PETER HAYES Phase-equilibrium data and liquidus isotherms for the system MnO -CaO-(Al 2 O 3 SiO 2 ) at silicomanganese alloy saturation have been determined in the temperature range of 1373 to 1723 K. The results are presented in the form of the pseudoternary sections MnO -CaO-(Al 2 O 3 SiO 2 ) with Al 2 O 3 /SiO 2 weight ratios of 0.55 and The primary-phase fields have been identified in this range of conditions. I. INTRODUCTION THE principal components of slags produced during the silicomanganese and high-carbon ferromanganese smelting processes are MnO, CaO, MgO, Al 2 O 3, and SiO 2. The relative proportion of each oxide in the final slag depends on the compositions of the original ores and the plant practice adopted. The pseudoternary section of the form MnO - CaO-(Al 2 O 3 SiO 2 ) for an Al 2 O 3 /SiO 2 weight ratio of 0.41 has been recently reported by the present authors. [1] This system closely represents the silicomanganese smelting slags produced from the processing of ores with relatively low magnesia contents. Silicomanganese slags usually contain between 15 and 20 wt pct MnO and have a low Al 2 O 3 /SiO 2 weight ratio, whereas ferromanganese slags contain 30 to 45 wt pct MnO and have a high Al 2 O 3 /SiO 2 weight ratio. The present study has been conducted to provide information on the liquidus temperatures in ferromanganese smelting slags relevant to process operations at TEMCO s Georgetown plant in Tasmania. The quaternary system consists of six binary and four ternary systems. The information available on these binary and ternary systems has been reviewed in a previous publication. [1] The liquidus for the CaO-Al 2 O 3 -SiO 2 join is taken from Eriksson and Pelton, [2] and that for the MnO -Al 2 O 3 - SiO 2 is taken from a recent study by the present authors. [3] In the investigation described subsequently, the phase equilibria and the liquidus in the pseudoternary sections of the form MnO -CaO-(Al 2 O 3 SiO 2 ) for Al 2 O 3 /SiO 2 weight ratios of 0.55 and 0.65 are presented. II. EXPERIMENTAL The general experimental procedures used in the present investigation have been described in a previous article. [1] The mixtures of slag and manganese-silicon alloy were held in a container of molybdenum foil in ultrahigh-purity nitrogen. The temperature accuracy was ensured to be within 5 K. After equilibration at the desired temperatures, the samples were then quenched rapidly to room temperature GHASEM ROGHANI, formerly Postdoctoral Research Fellow, EVGUENI JAK, Research Director, and PETER HAYES, Director and Professor, are with PYROSEARCH, Pyrometallurgy Research Centre, School of Engineering, The University of Queensland, St Lucia, Queensland, 4072, Australia. Contact e.jak@minmet.uq.edu.au Manuscript submitted August 23, and, subsequently, mounted and polished for further examination. On quenching, the liquid phase is retained as a glass. The microstructures and phases present in quenched slag samples were first examined using the optical microscope. The phases present were then identified by electron probe X-ray microanalysis (EPMA) using a JEOL* 8800L instru- *JEOL is a trademark of Japan Electron Optics Ltd., Tokyo. ment with wavelength-dispersive spectrometers. A rhodonite (MnSiO 3 ) standard was used for manganese, a spessartine (3MnO Al 2 O 3 3SiO 2) standard was used for aluminium, and a wollastonite (CaSiO 3 ) standard was used for silicon and calcium calibration. These standards were obtained from the Charles M. Taylor Company. The Duncumb-Philibert ZAF correction procedure supplied with the JEOL 8800L was applied. The average accuracy of the EPMA measurements was within 1 wt pct. III. RESULTS A. Pseudoternary Section MnO -CaO-(Al 2 O 3 SiO 2 ) at Al 2 O 3 /SiO The experimentally determined liquidus surface for the pseudoternary section of MnO -CaO-(Al 2 O 3 SiO 2 ) with an Al 2 O 3 /SiO 2 weight ratio of 0.55 is presented in Figures 1 through 3. Figure 1 shows the data points obtained in the present investigation. Figures 2 and 3 provide more detailed views of the section in the high-sio 2, and low-cao regions of the system, respectively. The chemical compositions of the liquid and solid phases determined in this study are given in Table I. The liquidus for the CaO-Al 2 O 3 -SiO 2 and MnO - Al 2 O 3 -SiO 2 sections is taken from previous studies. [2,3] The pseudoternary system MnO -CaO-(Al 2 O 3 SiO 2 ) with an Al 2 O 3 /SiO 2 weight ratio of 0.55 contains the primaryphase fields of mullite (3Al 2 O 3 2SiO 2, or A 3 S 2 ), corundum (Al 2 O 3, or A), spessartine (3MnO Al 2 O 3 3SiO 2, or M 3 AS 3 ), galaxite (MnO Al 2 O 3, or MA), manganosite solid solution ((Mn,Ca)O), a-dicalcium silicate (2CaO SiO 2, or C 2 S), a dicalcium silicate (2CaO SiO 2, or C 2 S), gehlenite (2CaO Al 2 O 3 SiO 2 or C 2 AS), and anorthite (CaO Al 2 O 3 2SiO 2, or CAS 2 ). B. Pseudoternary Section MnO -CaO-(Al 2 O 3 SiO 2 ) at Al 2 O 3 /SiO The experimentally determined liquidus surfaces and liquidus isotherms for the pseudoternary section of MnO - CaO-(Al 2 O 3 SiO 2 ) with an Al 2 O 3 /SiO 2 weight ratio of METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 34B, APRIL

2 Fig. 1 Liquidus in the MnO -CaO-(Al 2 O 3 SiO 2 ) system with Al 2 O 3 /SiO 2 weight ratio of 0.55 in equilibrium with Mn -Si alloy. Fig. 2 Liquidus in the MnO -CaO-(Al 2 O 3 SiO 2 ) system with Al 2 O 3 / SiO 2 weight ratio of 0.55 in equilibrium with Mn -Si Alloy (low CaO region and low MnO region). Fig. 3 Liquidus in the MnO -CaO-(Al 2 O 3 SiO 2 ) system with Al 2 O 3 / SiO 2 weight ratio of 0.55 in equilibrium with Mn -Si alloy (low CaO region and high MnO) are presented in Figures 4 through 6. Figure 4 shows the liquidus data points obtained in the present investigation. Figures 5 and 6 provide more detailed views of the section in the high-sio 2 and low-cao regions of the system, respectively. The chemical compositions of the liquid and solid phases determined in this study are given in Table II. 174 VOLUME 34B, APRIL 2003 METALLURGICAL AND MATERIALS TRANSACTIONS B

3 Table 1. Experimental Data for the MnO -CaO-Al 2 O 3 -SiO 2 System in Equilibrium with Mn/Si Alloy for the Al 2 O 3 /SiO 2 Weight Ratio of 0.55 Experiment Phases in Phase MnO CaO SiO 2 Al 2 O 3 ID T (K) Equilibrium* Name* (Wt Pct) (Wt Pct) (Wt Pct) (Wt Pct) Al 2 O 3 /SiO 2 Equilibria (Liquid Corundum Al 2 O 3 Mn/Si Alloy) C L C L C Equilibria (Liquid Mullite A3S2 Mn/Si Alloy) C L A3S2 L A3S C L A3S2 L A3S Equilibria (Liquid Anorthite CAS2 Mn/Si Alloy) C L CAS2 L CAS Q L CAS2 L CAS C L CAS2 L CAS C L CAS2 L CAS C L CAS2 L CAS C L CAS2 L CAS Equilibria (Liquid Galaxite MA Mn/Si Alloy) C L MA L MA C L MA L MA C L MA L MA C L MA L MA C L MA L MA C L MA L MA C L MA L MA C L MA L MA C L MA L MA Equilibria (Liquid Anorthite CAS2 Corundum C Mn/Si Alloy) C L CAS2 C L CAS C Equilibria (Liquid Galaxite MA Anorthite CAS2 Mn/Si Alloy) C L MA CAS2 L MA CAS C L MA CAS2 L MA CAS C L MA CAS2 L MA CAS C L MA CAS2 L MA CAS METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 34B, APRIL

4 Table 1. Continued. Experimental Data for the MnO -CaO-Al 2 O 3 -SiO 2 System in Equilibrium with Mn/Si Alloy for the Al 2 O 3 /SiO 2 Weight Ratio of 0.55 Experiment Phases in Phase MnO CaO SiO 2 Al 2 O 3 ID T (K) Equilibrium* Name* (Wt Pct) (Wt Pct) (Wt Pct) (Wt Pct) Al 2 O 3 /SiO 2 C L MA CAS2 L MA CAS C L MA CAS2 L MA CAS Equilibria (Liquid Galaxite MA Manganosite M Mn/Si Alloy) C L MA M L MA M C L MA M L MA M Equilibria (Liquid Gehlenite C2AS Manganosite M Mn/Si Alloy) C L C2AS M L C2AS M C L C2AS M L C2AS M Equilibria (Liquid Mullite A3S2 Corundum C Mn/Si Alloy) C L A3S2 C L A3S C The pseudoternary system MnO -CaO-(Al 2 O 3 SiO 2 ) with an Al 2 O 3 /SiO 2 weight ratio of 0.65 contains the primaryphase fields of mullite (3Al 2 O 3 2SiO 2, or A 3 S 2 ), corundum (Al 2 O 3, or A), galaxite (MnO Al 2 O 3, or MA), manganosite solid solution ((Mn,Ca)O), a-dicalcium silicate (2CaO SiO 2, or C 2 S), a -dicalcium silicate (2CaO SiO 2, or C 2 S), gehlenite (2CaO Al 2 O 3 SiO 2, or C 2 AS), and anorthite (CaO Al 2 O 3 2SiO 2, or CAS 2 ). The primary-phase fields shown in Figures 1 through 6 are separated by boundary curves, with arrows indicating the falling temperature. Liquidus isotherms have been determined in the present study between 1473 and 1673 K (1200 C to 1400 C) for the system MnO -CaO-(Al 2 O 3 SiO 2 ) with Al 2 O 3 /SiO 2 weight ratios of 0.55 and 0.65 and are shown with fine lines. Those phase boundaries marked with dashed lines have been marked following interpolation from available experimental data and require further work to confirm their exact positions. Data for Al 2 O 3 /SiO 2 weight ratios not exactly on the sections are not included in the figures, but these data have been retained in Tables I and II because of their usefulness for thermodynamic modeling. IV. DISCUSSION The primary-phase fields and the general shapes of the primary-phase fields for Al 2 O 3 /SiO 2 weight ratios of 0.55 and 0.65 are similar to those for an Al 2 O 3 /SiO 2 weight ratio of [1] The sections are characterized by high-meltingtemperature phases, on the one hand, along the MnO-CaO join and, on the other hand, to the (Al 2 O 3 SiO 2 ) apex of the sections. Between these extremes is a lowmelting-temperature region extending from approximately the midpoint of the MnO-(Al 2 O 3 SiO 2 ) join toward the CaO-(Al 2 O 3 SiO 2 ) join. The middle of the CaO-(Al 2 O 3 SiO 2 ) join is dominated by the primary-phase fields of anorthite (CAS 2 ) and gehlenite (C 2 AS), the low-meltingtemperature region becoming confined and bounded by the anorthite- and gehlenite-phase fields as the MnO concentration is progressively reduced. Figures 7 and 8 illustrate the changes of the liquidus temperature with MnO concentration for the system MnO - CaO-(Al 2 O 3 SiO 2 ) for CaO/(Al 2 O 3 SiO 2 ) weight ratios of 0.1, 0.25, 0.5, and 0.75 and for Al 2 O 3 /SiO 2 weight ratios of 0.55 and In all of the aforementioned sections, starting at high MnO concentrations, the liquidus decreases as the MnO concentration is lowered in the manganosite primary-phase field; the liquidus moves through a minimum value before rising again at lower MnO concentrations. The minimum liquidus temperature increases with increasing CaO/(Al 2 O 3 SiO 2 ) weight ratio. Figures 9 and 10 show the liquidus surfaces for the two sections at fixed MnO concentrations and increasing CaO concentrations. It is that the liquidus temperatures of slags containing 40 wt pct MnO are significantly lower than those containing 10 and 20 wt pct MnO. Another interesting feature of the system that can be seen from Figures 7 and 8 is that, in the manganosite primaryphase field for a given MnO concentration, increasing the CaO/(Al 2 O 3 SiO 2 ) weight ratio dramatically increases the liquidus temperature and decreases the MnO concentration in the slag at manganosite saturation. This is significant, 176 VOLUME 34B, APRIL 2003 METALLURGICAL AND MATERIALS TRANSACTIONS B

5 Fig. 4 Liquidus in the MnO -CaO-(Al 2 O 3 SiO 2 ) system with Al 2 O 3 /SiO 2 weight ratio of 0.65 in equilibrium with Mn -Si alloy. Fig. 5 Liquidus in the MnO -CaO-(Al 2 O 3 SiO 2 ) system with Al 2 O 3 / SiO 2 weight ratio of 0.65 in equilibrium with Mn -Si alloy (low CaO region and low MnO region). Fig. 6 Liquidus in the MnO -CaO-(Al 2 O 3 SiO 2 ) system with Al 2 O 3 / SiO 2 weight ratio of 0.65 in equilibrium with Mn -Si alloy (low CaO region and high MnO). because in ferromanganese production it is desirable to maintain a high MnO activity in the slag even to low MnO concentrations. As evidence in support of this contention, Rait and Olsen [4] cited kinetic studies of Olso et al., [5] who noted a distinct reduction in the rate of reduction of ferromanganese slags as the slags depart from manganosite saturation. The EPMA measurements of the chemical composition of solid phase in the manganosite primary- METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 34B, APRIL

6 Table II. Experimental Data for the MnO -CaO-Al 2 O 3 -SiO 2 System in Equilibrium with Mn/Si Alloy for the Al 2 O 3 /SiO 2 Weight Ratio of 0.65 Experiment Phases in Phase MnO CaO SiO 2 Al 2 O 3 ID T (K) Equilibrium* Name* (Wt Pct) (Wt Pct) (Wt Pct) (Wt Pct) Al 2 O 3 /SiO 2 Equilibria (Liquid Corundum C Mn/Si Alloy) C L C L C C L C L C C L C L C Equilibria (Liquid Gehlenite C2AS Mn/Si Alloy) C L C2AS L C2AS C L C2AS L C2AS Equilibria (Liquid Galaxite MA Mn/Si Alloy) C L MA L MA C L MA L MA C L MA L MA C L MA L MA C L MA L MA Equilibria (Liquid Manganosite M Mn/Si Alloy) C L M L M C L M L M C L M L M C L M L M Equilibria (Liquid Anorthite CAS2 Mn/Si Alloy) C L CAS2 L CAS C L CAS2 L CAS C L CAS2 L CAS C L CAS2 L CAS C L CAS2 L CAS Q L CAS2 L CAS C L CAS2 L CAS C L CAS2 L CAS Equilibria (Liquid Anorthite CAS2 Corundum C Mn/Si Alloy) C L CAS2 C L CAS C C L CAS2 C L CAS C C L CAS2 C L CAS C VOLUME 34B, APRIL 2003 METALLURGICAL AND MATERIALS TRANSACTIONS B

7 Table II. Continued. Experimental Data for the MnO -CaO-Al 2 O 3 -SiO 2 System in Equilibrium with Mn/Si Alloy for the Al 2 O 3 /SiO 2 Weight Ratio of 0.65 Experiment Phases in Phase MnO CaO SiO 2 Al 2 O 3 ID T (K) Equilibrium* Name* (Wt Pct) (Wt Pct) (Wt Pct) (Wt Pct) Al 2 O 3 /SiO 2 Equilibria (Liquid Gehlenite C2AS Manganosite M Mn/Si Alloy) C L C2AS M L C2AS M Equilibria (Liquid Galaxite MA Anorthite CAS2 Mn/Si Alloy) C L MA CAS2 L MA CAS Equilibria (Liquid Galaxite MA Manganosite M Mn/Si Alloy) C L MA M L MA M C L MA M L MA M *List of phases observed in present experimental program: Name Chemical Formula Abbreviation used in Tables Liquid L Corundum Al 2 O 3 C Manganosite (Mn,Ca)O M Galaxite MnAl 2 O 4 MA Tridymite SiO 2 S Cristabolite SiO 2 S Mullite Al 6 Si 2 O 13 A3S2 Rhodonite MnSiO 3 MS Tephroite Mn 2 SiO 4 M2S Gehlenite Ca 2 Al 2 SiO 7 C2AS Anorthite CaAl 2 Si 2 O 8 CAS2 Mn-Al-silicate MnAl 2 Si 2 O 8 MAS2 Spessartine Mn 3 Al 2 Si 3 O 12 M3AS3 phase field (Tables I and II) show that it consists of almost pure MnO; the CaO content in the solid phase is low and increases only marginally with increasing CaO content in the liquid slag phase. For example, at 1573 K at an Al 2 O 3 / SiO 2 weight ratio of 0.65 for the liquid composition 38.8 wt pct MnO, 22.0 wt pct CaO, and 23.8 wt pct SiO 2, the manganosite contains 98.1 wt pct MnO and 1.3 wt pct CaO. The liquidus isotherms (1523 to 1773 K) for the manganosite primary-phase field for an Al 2 O 3 /SiO 2 weight ratio of 0.55 for the pseudoternary MnO -Al 2 O 3 -SiO 2, measured in a recent study by the present authors, [3] are shown in Figure 11. Also included in this figure are the experimentally determined liquidus data obtained by Rait and Olsen [4] for Al 2 O 3 /SiO 2 weight ratios between 0.44 and 0.55, for temperatures between 1723 and 1823 K. It can be seen that the experimental data on the quaternary systems generated by Rait and Olsen are consistent with the new ternary data. [3] On a weight basis, the isotherms are almost parallel to the join of CaO-MnO in the range of conditions investigated. In constructing the liquidus isotherms, Rait and Olsen had previously assumed the liquidus for the system MnO - Al 2 O 3 -SiO 2 as reported by Muan and Osborn [6] to be correct, and, hence, the isotherms appeared to exhibit unusually sharp curvature as the ternary is approached. The results of the investigations by the present authors [3] resolve the apparent anomaly. V. CONCLUSIONS The liquidus isotherms have been determined experimentally in the range of 1423 to 1623 K at intervals of 50 K and are presented on the pseudoternary sections MnO - CaO-(Al 2 O 3 SiO 2 ) with Al 2 O 3 /SiO 2 weight ratios of 0.55 and 0.65 in equilibrium with silicomanganese alloy saturation. The major primary fields for the pseudoternary sections of MnO -CaO-Al 2 O 3 -SiO 2 with Al 2 O 3 /SiO 2 weight ratios of 0.55 and 0.65 are mullite (3Al 2 O 3 2SiO 2 ), corundum (Al 2 O 3 ), galaxite (MnO Al 2 O 3 ), manganosite (MnO), tephroite (2MnO SiO 2 ), anorthite (CaO Al 2 O 3 2SiO 2 ), a-dicalcium silicate (2CaO SiO 2, or C 2 S), a -dicalcium silicate (2CaO SiO 2, or C 2 S), and gehlenite (2CaO Al 2 O 3 SiO 2 ) in the composition range investigated. The pseudoternary section with an Al 2 O 3 /SiO 2 weight ratio of 0.55 also contains the primary-phase field of the spessartine (3MnO Al 2 O 3 3SiO 2, or M 3 AS 3 ). The liquidus isotherms for the manganosite primary field, which are of particular importance to ferromanganese smelting practice, are shown to lie almost parallel to the join CaO-MnO when viewed on a weight percent basis. Increasing the CaO/(Al 2 O 3 SiO 2 ) weight ratio for a given MnO concentration dramatically increases the liquidus temperature. METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 34B, APRIL

8 Fig. 7 Liquidus in the MnO -CaO-(Al 2 O 3 SiO 2 ) system at Al 2 O 3 /SiO 2 weight ratio of 0.55 in equilibrium with Mn -Si alloy, as a function of MnO concentration for CaO/(Al 2 O 3 SiO 2 ) 0.1, 0.25, 0.5, and Fig. 8 Liquidus in the MnO -CaO-(Al 2 O 3 SiO 2 ) system at Al 2 O 3 /SiO 2 weight ratio of 0.65 in equilibrium with Mn -Si alloy, as a function of MnO concentration for CaO/(Al 2 O 3 SiO 2 ) 0.1, 0.25, 0.5, and VOLUME 34B, APRIL 2003 METALLURGICAL AND MATERIALS TRANSACTIONS B

9 Fig. 9 Liquidus in the MnO -CaO-(Al 2 O 3 SiO 2 ) system at Al 2 O 3 /SiO 2 weight ratio of 0.55 in equilibrium with Mn -Si alloy, as a function of CaO concentration for MnO 10, 20, and 40 wt pct. Fig. 10 Liquidus in the MnO -CaO-(Al 2 O 3 SiO 2 ) system at Al 2 O 3 /SiO 2 weight ratio of 0.65 in equilibrium with Mn -Si alloy, as a function of CaO concentration for MnO 10, 20, and 40 wt pct. METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 34B, APRIL

10 ACKNOWLEDGMENTS The authors thank BHP Billiton Temco (Georgetown plant in Tasmania, Australia), for providing financial support for this project, in particular, Dr. Samir Ganguly, Technical Superintendent, who helped to initiate the study. Thanks also to the staff of the Centre for Microanalysis and Microscopy, University of Queensland, for providing support for the electron-probe X-ray microanalysis measurements. Fig. 11 Liquidus isotherms in the manganosite primary phase field for MnO -CaO-(Al 2 O 3 SiO 2 ) system at Al 2 O 3 /SiO 2 weight ratio of 0.44 to 0.55 in equilibrium with Mn -Si alloy. Data from Rait and Olsen [4] and Roghani et al. [3] REFERENCES 1. G. Roghani, E. Jak, and P. Hayes: Metall. Mater. Trans. B, 2002, vol. 33B, pp G. Eriksson and A.D. Pelton: Metall. Trans. B, 1993, vol. 24B, pp G. Roghani, E. Jak, and P. Hayes: Metall. Mater. Trans. B, 2002, vol. 33B, pp R. Rait and S.E. Olsen: Scand. J. Metall., 1999, vol. 28, pp V. Olso, M. Tangsted, and S.E. Olsen: 8th Int. Ferroalloys Congr. Proc., Beijing, China Science & Technology Press, 1998, pp E.F. Osborn and A. Muan: Phase Equilibria among Oxides in Steelmaking, Addison-Wesley Publishing Company, Inc., New York, NY, 1965, Fig. 109, reproduced in Slag Atlas, Verlag Stahleisen, Dusseldorf, Germany, 1995, Fig VOLUME 34B, APRIL 2003 METALLURGICAL AND MATERIALS TRANSACTIONS B