thermodynamic models and databases for molten salts and slags

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Thermodynamic Models and Databases for Molten Salts and Slags. Model parameters obtained by simultaneous evaluation/optimization of thermodynamic and phase equilibrium data for 2-component and, if available, 3-component systems. Model parameters stored in databases - PowerPoint PPT Presentation

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  • Thermodynamic Models and Databases for Molten Salts and SlagsArthur PeltonCentre de Recherche en Calcul Thermochimiquecole Polytechnique, Montral, CanadaModel parameters obtained by simultaneous evaluation/optimization of thermodynamic and phase equilibrium data for 2-component and, if available, 3-component systems.Model parameters stored in databasesModels used to predict properties of N-component salts and slagsWhen combined with databases for other phases (gas, metal, etc.) can be used to calculate complex multi-phase, multi-component equilibria using Gibbs energy minimization software.

  • Reciprocal molten salt system Li,K/F,ClLiquidus projection

  • Section of the preceding phase diagramalong the LiF-KCl diagonalA tendency to de-mixing (immiscibility) is evident.This is typical of reciprocal salt systems, many of which exhibit an actual miscibility gap oriented along one diagonal.

  • Molecular ModelRandom mixture of LiF, LiCl, KF and KCl molecules.

    Exchange Reaction: LiCl + KF = LiF + KCl DGEXCHANGE < OTherefore, along the LiF-KCl stable diagonal, the model predicts an approximately ideal solution of mainly LiF and KCl molecules.Poor agreement with the observed liquidus.

  • Random Ionic (Sublattice) ModelRandom mixture of Li+ and K+ on cationic sublattice and of F- and Cl- on anionic sublattice.

    Along the stable LiF-KCl diagonal, energetically unfavourable Li+- Cl- and K+- F- nearest-neighbour pairs are formed. This destabilizes the solution and results in a tedency to de-mixing (immiscibility) that is, a tedency for the solution to separate into two phases: a LiF-rich liquid and a KCl-rich liquid.This is qualitatively correct, but the model overestimates the tedency to de-mixing.

  • Ionic Sublattice Model with Short-Range-OrderingBecause Li+- F- and K+- Cl- nearest-neighbour are energetically favoured, the concentrations of these pairs in solution are greater than in a random mixture:Number of Li+- F- pairs = (XLiXF + y) Number of K+- Cl- pairs = (XKXCl + y) Number of Li+- Cl- pairs = (XLiXCl - y) Number of K+- F- pairs = (XKXF - y) Exchange Reaction:LiCl + KF = LiF + KClThis gives a much improved prediction.

  • For quantitative calculations we must also take account of deviations from ideality in the four binary solutions on the edges of the composition square.For example, in the LiF-KF binary system, an excess Gibbs energy term , GE, arises because of second-nearest-neighbour interactions: (Li-F-Li) + (K-F-K) = 2(Li-F-K) (Generally, these GE terms are negative: .) is modeled in the binary system by fitting binary data.In predicting the effect of within the reciprocal system, we must calculate the probability of finding an (Li-F-K) second-nearest-neighbour configuration, taking account of the aformentioned clustering of Li+- F- and K+- Cl- pairs. Account should also be taken of second-nearest-neighbour short-range-ordering.

  • Liquidus projection calculated from the quasichemical model in the quadruplet approximation (P. Chartrand and A. Pelton)

  • Experimental (S.I. Berezina, A.G. Bergman and E.L. Bakumskaya) liquidus projection of the Li,K/F,Cl system

  • Phase diagram section along the LiF-KCl diagonal The predictions are made solely from the GE expressions for the 4 binary edge systems and from DGEXCHANGE. No adjustable ternary model parameters are used.

  • SILICATE SLAGSThe basic region (outlined in red) is similar to a reciprocal salt system, with Ca2+ and Mg2+ cations and, to a first approximation, O2- and (SiO4)4- anions.The CaO-MgO-SiO2 phase diagram.

  • Exchange Reaction: Mg2(SiO4) + 2 CaO = Ca2(SiO4) + 2 MgO DGEXCHANGE < O

    Therefore there is a tedency to immiscibility along the MgO-Ca2(SiO4) join as is evident from the widely-spaced isotherms.

  • Associate ModelsModel the MgO-SiO2 binary liquid assuming MgO, SiO2 and Mg2SiO4 moleculesWith the model parameter DG< 0, one can reproduce the Gibbs energy of the binary liquid reasonably well:Gibbs energy of liquid MgO-SiO2 solutions

  • The CaO-SiO2 binary is modeled similarly.

    Since DGEXCHANGE < 0, the solution along the MgO-Ca2SiO4 join is modeled as consisting mainly of MgO and Ca2SiO4 molecules.

    Hence the tendency to immiscibility is not predicted.

  • Reciprocal Ionic Liquid Model(M. Hillert, B. Jansson, B. Sundman, J. Agren)

    Ca2+ and Mg2+ randomly distributed on cationic sublatticeO2-, (SiO4)4- and neutral SiO2 species randomly distributed on anionic sublatticeAn equilibrium is established:(Very similar to: O0 + O2- = 2 O-)

    In basic melts mainly Ca2+, Mg2+, O2-, (SiO4)4- randomly distributed on two sublattices. Therefore the tendency to immiscibility is predicted but is overestimated because short-range-ordering is neglected.

  • The effect of a limited degree of short-range-ordering can be approximated by adding ternary parameters such as:Very acid solutions of MO in SiO2 are modeled as mixtures of (SiO2)0 and (SiO4)4-

    Model has been used with success to develop a large database for multicomponent slags.

  • Modified Quasichemical ModelA. Pelton and M. BlanderQuasichemical reaction among second-nearest-neighbour pairs: (Mg-Mg)pair + (Si-Si)pair = 2(Mg-Si)pairDG < 0 (Very similar to: O0 + O2- = 2 O-)

    In basic melts:Mainly (Mg-Mg) and (Mg-Si) pairs (because DG < 0).That is, most Si atoms have only Mg ions in their second coordination shell.This configuration is equivalent to (SiO4)4- anions.In very basic (MgO-SiO2) melts, the model is essentially equivalent to a sublattice model of Mg2+, Ca2+, O2-, (SiO4)4- ions.

  • However, for the quasichemical exchange reaction: (Ca-Ca) + (Mg-Si) = (Mg-Mg) + (Ca-Si) DGEXCHANGE < 0

    Hence, clustering (short-range-ordering) of Ca2+-(SiO4)4- and Mg2+-O2- pairs is taken into account by the model without the requirement of ternary parameters.

    At higher SiO2 contents, more (Si-Si) pairs are formed, thereby modeling polymerization.Model has been used to develop a large database for multicomponent systems.

  • The Cell ModelM.L. Kapoor, G.M. Frohberg, H. Gaye and J. Welfringer

    Slag considered to consist of cells which mix essentially ideally, with equilibria among the cells: [Mg-O-Mg] + [Si-O-Si] = 2 [Mg-O-Si]DG < 0

    Quite similar to Modified Quasichemical ModelAccounts for ionic nature of slags and short-range-ordering.Has been applied with success to develop databases for multicomponent systems.

  • Liquidus projection of the CaO-MgO-SiO2-Al2O3 system at 15 wt % Al2O3, calculated from the Modified Quasichemical Model

  • Liquidus projection of the CaO-MgO-SiO2-Al2O3 system at 15 wt % Al2O3, as reported by E. Osborn, R.C. DeVries, K.H. Gee and H.M. Kramer

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