Coupled Experimental Study and Thermodynamic Optimization of the K 2 O-MgO and K 2 O-MgO-SiO 2 Systems
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INTRODUCTION
THE K2O-MgO-SiO2 system finds importance in various applications. In pyrometallurgical processes, magnesia (MgO) is the basis of the most commonly used refractories such as magnesia-carbon, dolomite, magnesia-spinel, etc.[1,2] SiO2 is the common component in slag, which is in direct contact with the refractories. In glassmaking, this ternary system is of interest for new silica-based glass optical fibers and mica/cordieritebased glass–ceramics.[3–6] The compounds such as K2MgSiO4 and K2MgSi3O8 are considered to be good candidates for fertilizer due to slow-releasing potential of K, Mg, and Si for crops.[7,8] In geological studies, the phase stability and thermodynamic properties of the compounds in this system are essential to understand the complex phase equilibria in multicomponent systems such as K2O-MgO-Al2O3-SiO2, which involve many mineralogically important phases like cordierite, leucite, potash feldspar, etc.[9] Therefore, accurate phase diagram and thermodynamic information in the K2O-MgO-SiO2 system are important. In the thermodynamic optimization, all the thermodynamic and phase equilibrium data available in the literature DONG-GEUN KIM, BIKRAM KONAR, and IN-HO JUNG are with the Department of Mining and Materials Engineering, McGill University, 3610 University St., Montreal, Quebec, Canada, H3A 0C5. Contact e-mail: [email protected] Manuscript submitted March 12, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
are simultaneously evaluated and optimized in thermodynamically consistent manner. As a result, the most reliable thermodynamic description of all phases is obtained. The resultant thermodynamic functions can be used to calculate any phase diagram and thermodynamic properties of the system, even those that have never been experimentally explored. There is a dearth of studies on thermodynamic property and phase diagram in the K2O-MgO and K2O-MgO-SiO2 systems. Until now, regarding the K2O-MgO system, only two studies[10,11] on the stability of K6MgO4 compound are available in the literature. For the K2O-MgO-SiO2 system, there have been two phase diagram studies by Roedder[9] and Roth[12] and several studies[9,12–16] on the structure of the ternary compounds. A thermodynamic optimization on the K2O-MgO-SiO2 system was performed by Yazhenskikh et al.[17] using the associate model[18] for the description of liquid phase. Even though the liquidus of the ternary system was generally well reproduced, there are some limitations in their optimization. Two stable binary compounds, K6MgO4 and K4SiO4, were neglected in their study. That is, the binary K2O-MgO and K2O-SiO2 systems were less satisfactory. Cristobalite (SiO2) appeared in the temperatures ranging from 1139.9 K to 1465.3 K (866.9 C to 1465.3 C), where tridymite (SiO2) should be the stable form of SiO2.[19] This leads to a significant alteration of the phase equilibria in the high SiO2 region of the system. In those authors’ optimization, the first-order polymorphic transition of K2MgSiO4 at 781 K (508 C) as reported by Dollase[16] w
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