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Thermodynamic Models and Databases for Molten Salts and Slags

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

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Thermodynamic Models and Databases for Molten Salts and Slags

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  1. 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 • Models used to predict properties of N-component salts and slags • When 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. Arthur Pelton Centre de Recherche en Calcul Thermochimique École Polytechnique, Montréal, Canada

  2. Reciprocal molten salt system Li,K/F,Cl • Liquidus projection

  3. Section of the preceding phase diagramalong the LiF-KCl diagonal • A 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.

  4. Molecular Model • Random mixture of LiF, LiCl, KF and KCl molecules. • Exchange Reaction:LiCl + KF = LiF + KClDGEXCHANGE < O • Therefore, 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.

  5. Random Ionic (Sublattice) Model • Random 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.

  6. Because 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 + KCl This gives a much improved prediction. Ionic Sublattice Model with Short-Range-Ordering

  7. 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.

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

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

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

  11. SILICATE SLAGS • The CaO-MgO-SiO2 phase diagram. • The 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.

  12. 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.

  13. Associate Models • Model the MgO-SiO2 binary liquid assuming MgO, SiO2 andMg2SiO4 «molecules» • With the model parameter DG°< 0, one can reproduce the Gibbs energy of the binary liquid reasonably well: Gibbs energy of liquid MgO-SiO2 solutions

  14. 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.

  15. (M. Hillert, B. Jansson, B. Sundman, J. Agren) Ca2+ and Mg2+ randomly distributed on cationic sublattice O2-, (SiO4)4- and neutral SiO2 species randomly distributed on anionic sublattice An 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. Reciprocal Ionic Liquid Model

  16. 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.

  17. Modified Quasichemical Model A. Pelton and M. Blander • «Quasichemical» reaction among second-nearest-neighbour pairs: (Mg-Mg)pair + (Si-Si)pair = 2(Mg-Si)pair DG° < 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.

  18. 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.

  19. The Cell Model M.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 Model • Accounts for ionic nature of slags and short-range-ordering. • Has been applied with success to develop databases for multicomponent systems.

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

  21. 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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