# thermodynamic models and databases for molten salts and slags

Post on 05-Feb-2016

35 views

Embed Size (px)

DESCRIPTION

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 PresentationTRANSCRIPT

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

Recommended