Thermodynamic Models and Databases for Molten Salts and Slags

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Thermodynamic Models and Databases for Molten
Salts and Slags

• Arthur Pelton
• Centre de Recherche en Calcul Thermochimique
• École Polytechnique, Montréal, Canada

• 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.

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Reciprocal molten salt system Li,K/F,Cl

• Liquidus projection

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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.

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Molecular Model

• Random mixture of LiF, LiCl, KF and KCl
  molecules.
• Exchange Reaction LiCl KF LiF
  KCl DGEXCHANGE lt 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.

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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.

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Ionic Sublattice Model with Short-Range-Ordering

• 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.

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• 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.

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Liquidus projection calculated from the
quasichemical model in the quadruplet
approximation (P. Chartrand and A. Pelton)
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Experimental (S.I. Berezina, A.G. Bergman and
E.L. Bakumskaya) liquidus projection of the
Li,K/F,Cl system
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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.

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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.

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• Exchange Reaction
• Mg2(SiO4) 2 CaO Ca2(SiO4) 2 MgO
• DGEXCHANGE lt O
• Therefore there is a tedency to immiscibility
  along the MgO-Ca2(SiO4) join as is evident from
  the widely-spaced isotherms.

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Associate Models

• Model the MgO-SiO2 binary liquid assuming MgO,
  SiO2 and Mg2SiO4 molecules

• With the model parameter DGlt 0, one can
  reproduce the Gibbs energy of the binary liquid
  reasonably well

Gibbs energy of liquid MgO-SiO2 solutions
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• The CaO-SiO2 binary is modeled similarly.
• Since DGEXCHANGE lt 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.

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Reciprocal Ionic Liquid Model

• (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.Therefor
  e the tendency to immiscibility is predicted but
  is overestimated because short-range-ordering is
  neglected.

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• 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.

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Modified Quasichemical Model

• A. Pelton and M. Blander
• Quasichemical reaction among second-nearest-neig
  hbour pairs
• (Mg-Mg)pair (Si-Si)pair 2(Mg-Si)pair
• DG lt 0
• (Very similar to O0 O2- 2 O-)
• In basic melts
• Mainly (Mg-Mg) and (Mg-Si) pairs (because DG lt
  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.

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• However, for the quasichemical exchange
  reaction
• (Ca-Ca) (Mg-Si) (Mg-Mg) (Ca-Si)
• DGEXCHANGE lt 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.

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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 lt 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.

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

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• 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