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ISIJ International, Vol. 46 (2006), No. 11, pp. 1594–1602 Phase Equilibria in the System “MnO”–CaO–MgO–SiO2–Al2O3 with Al2O3/SiO2 Weight Ratio of 0.17 and MgO/CaO Weight Ratio of 0.25 at Mn–Si Alloy Saturation Baojun ZHAO, Eugene JAK and Peter C. HAYES Pyrometallurgy Research Centre, School of Engineering, The University of Queensland, Brisbane, Queensland, 4072, Australia. E-mail: [email protected] (Received on July 14, 2006; accepted on September 5, 2006) Liquidus isotherms and phase equilibria have been determined experimentally for a pseudo-ternary sec- ϩ ϩ tion of the form “MnO”–(CaO MgO)–(SiO2 Al2O3) with a fixed Al2O3/SiO2 weight ratio of 0.17 and MgO/CaO weight ratio of 0.25 for temperatures in the range 1 393–1 673 K. The primary phase fields found in the investigated section include manganosite (Mn, Mg, Ca)O; dicalcium silicate a-2(Ca, Mg, Mn)O · SiO2; merwinite 3CaO · (Mg, Mn)O · 2SiO2; melilite [2CaO · (MgO, MnO, Al2O3)· 2(SiO2,Al2O3)]; wollastonite [(Ca, Mg, Mn)O · SiO2]; diopside [(CaO, MgO, MnO, Al2O3)·SiO2]; tridymite (SiO2); rhodonite [(Mn, Mg, Ca)O · SiO2]; anorthite (CaO · Al2O3 · 2SiO2) and tephroite [2(Mn, Mg, Ca)O · SiO2]. The liquidus temperatures and primary phase fields are significantly different to those in the ternary sys- tem “MnO”–CaO–SiO2, but are close to those previously reported pseudo-ternary section “MnO”– ϩ ϩ (CaO MgO)–(SiO2 Al2O3) for Al2O3/SiO2 weight ratio of 0.17 and MgO/CaO weight ratio of 0.17. The partitioning of CaO, MgO and MnO between liquid and solid phases was measured using EPMA, and the extents of solid solutions for a range of bulk compositions and temperatures were characterised. KEY WORDS: slag; equilibria; “MnO”–CaO–MgO–SiO2–Al2O3; liquidus. 1. Introduction 2. Experimental Procedure In a recent study by the authors1) the liquidus temper- The experimental method used in the present study is atures and phase equilibria in the quinary system identical to that reported by the authors in Part 1 of a series 1) “MnO”–CaO–MgO–SiO2–Al2O3 at Mn–Si alloy saturation of papers describing this investigation. The method essen- ϭ ϭ were reported for Al2O3/SiO2 0.17 and MgO/CaO 0.17. tially involves the preparation of mixtures from high purity That study demonstrated that the liquidus temperatures and oxide powders with excess of the Mn–Si alloy, high temper- primary phase fields in the quinary system are markedly ature equilibration in Ar gas atmosphere, rapid quenching different from those observed in the ternary system of the samples into cool water, and examination and analy- 2) “MnO”–CaO–SiO2 under reducing conditions. The liq- sis of the resulting microstructures and phase compositions uidus temperatures were shown to be principally dependent using electron probe X-ray microanalysis (EPMA). ϩ ϩ ϩ on the modified basicity weight ratio (CaO MgO)/(SiO2 The metal oxide powders used in the study are 99 % Al2O3) at low “MnO” concentrations, and dependent on the MnO, CaCO3, SiO2 and Al2O3. The Mn–Si alloy was pre- ϩ ϩ ϩ mole ratio (CaO MgO “MnO”)/(SiO2 Al2O3) at higher pared by mixing Mn and Si powders with mole ratio of “MnO” concentrations. Mn : Siϭ3:1 and heating under an argon gas atmosphere in In commercial ferro-manganese and silico-manganese a carbon crucible at 1 473 K for 2 h. Approximately 0.3 g of smelting practice the MgO/CaO ratio in the final slag is sig- powdered oxide/alloy mixture was placed in an envelope nificantly influenced by the MgO present in the feed to the made from 0.025 mm thick Mo metal foil, and suspended in furnace. High MgO slags are formed during pyrometallur- flowing stream of high purity Ar gas in the equilibration gical processing of South African manganese ore concen- furnace. The oxygen was removed from the alumina reac- trates. The present paper provides additional experimentally tion tube by flushing with the Ar, prior to lifting the sam- determined data on the phase equilibria and liquidus tem- ples suspended by Mo wire into the hot zone of the furnace. peratures for the quinary system “MnO”–CaO–MgO–SiO2– The presence of Mn–Si alloy particles distributed through- ϭ ϭ A12O3 for Al2O3/SiO2 0.17 and MgO/CaO 0.25, no data out the samples ensured that local equilibrium between slag in this composition range are available from the previous and metal was attained, and that the manganese in the slag studies on this system.3–8) was present principally as Mn2ϩ. The quenched samples were mounted and polished for © 2006 ISIJ 1594 ISIJ International, Vol. 46 (2006), No. 11 examination following high temperature equilibration. A the primary phase fields, with increasing (CaOϩMgO), ap- JEOL 8800L Electron Probe Microanalyser (EPMA) with pear in the sequence of cristobalite, tridymite, diopside, Wave Length Dispersive detectors was used for measure- melilite, merwinite, a-Ca2SiO4 and manganosite. ments of the phase compositions. The standards used for The primary phase fields observed in the section de- EPMA include alumina (Al2O3) for Al, magnesia (MgO) scribed in the present paper are the same as those observed ϭ ϭ for Mg, spessartine (Mn3Al2Si3O12) for Mn and wollas- for the section with Al2O3/SiO2 0.17 and MgO/CaO 0.17 tonite (CaSiO3) for Ca and Si. These standards were pro- but also include the anorthite primary phase field. vided by Charles M Taylor Co., Stanford, California, USA. The analysis was conducted at an accelerating voltage of 3.2. Comparison with Other Systems 15 kV and a probe current of 15 nA. The ZAF correction The experimental data on the liquidus obtained in the ϩ ϩ procedure supplied with the electron probe was applied. present study in the joins (CaO MgO)–(SiO2 Al2O3) with The average accuracy of the EPMA measurements is within fixed Al2O3/SiO2 weight ratio of 0.17 and MgO/CaO weight Ϯ1 wt%. The phase compositions were recalculated to ox- ratio of 0.25 in the liquid are in good agreement with those ides on the assumption that all manganese is present as interpolated from the system CaO–MgO–SiO2–Al2O3 re- Mn2ϩ. Although it is possible to measure the compositions ported by Osborn9) and Cavalier and Sandreo-Dendon.10) of the oxide phase unfortunately it has not been possible to Comparison of the phase diagram of the system 2) determine the corresponding compositions of the fine dis- “MnO”–CaO–SiO2 with the sections of the quinary sys- persed alloy in the quenched samples. tem containing MgO and Al2O3 determined in the present study indicates that, whilst the general position of the liq- uidus valley is similar in the ternary system, there are im- 3. Experimental Results portant differences in the primary phase fields and shape of 3.1. Description of the Pseudo-ternary Section the liquidus surface. The rankinite, aЈ-dicalcium silicate The compositions of the phases identified in the sam- and pseudo-wollastonite primary phase fields in the ternary 2) ples following equilibration of the oxide/alloy mixtures, system “MnO”–CaO–SiO2 are replaced by melilite, mer- measured using EPMA, are presented in Table 1. These winite, diopside, tephroite and anorthite in the sections of data have been used to construct liquidus isotherms on the quinary system. At the same time most importantly the ϩ ϩ the pseudo-ternary section “MnO”–(CaO MgO)–(SiO2 liquidus temperatures in the quinary system are signifi- Al2O3) with a fixed Al2O3/SiO2 weight ratio of 0.17 and cantly different from those in the ternary system. MgO/CaO weight ratio of 0.25. Figure 1 shows the experi- To assist in the comparison of the liquidus temperatures 2) mentally determined liquidus points for compositions hav- between the system ternary “MnO”–CaO–SiO2 and the ϩ ϩ ing the appropriate Al2O3/SiO2 and MgO/CaO ratios in the pseudo-ternary system “MnO”–(CaO MgO)–(SiO2 1) liquid phase. From these data the liquidus isotherms at Al2O3) determined in the present and previous studies, the 1 473, 1 523, 1 573, 1 623 and 1 673 K have been deter- liquidus for sections at 20 wt% “MnO” are shown in Fig. 3. mined. The remaining data with different Al2O3/SiO2 and In the ternary system primary phase fields of wollastonite, MgO/CaO ratios are also provided in Table 1 for subse- rankinite and dicalcium silicate are present in the composi- quent thermodynamic modelling of this system. The liq- tion range compared. In the two pseudo-ternary sections uidus surface of the pseudo-ternary section for the quinary melilite and tephroite are stable at low (CaOϩMgO) con- system is shown in Fig. 2. centrations and manganosite primary phase field is located The experimentally determined phase boundaries are at high (CaOϩMgO) concentrations. Between the tephroite marked with thick full lines. The boundaries marked with and manganosite primary phase fields, merwinite is present ϭ ϭ dashed lines indicate the approximate positions. Experi- in the section with Al2O3/SiO2 0.17 and MgO/CaO 0.25 mental data were obtained in all primary phase fields in and dicalcium silicate is present in the section with ϭ ϭ the pseudo-ternary section and the pseudo-binary joins Al2O3/SiO2 0.17 and MgO/CaO 0.17. ϩ ϩ ϩ ϩ (CaO MgO)–(SiO2 Al2O3) and “MnO”–(SiO2 Al2O3). It can be seen from Fig. 3 that at low (CaO MgO) con- The pseudo-ternary section is characterised by the centrations the liquidus temperatures in the quinary system presence of the following primary phase fields (see Fig. are significantly lower than those in the ternary in the com- 2): Manganosite (Mn, Mg, Ca)O; a-dicalcium silicate position range investigated.

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