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MTDATA Demonstration : Salt Systems
Contents
Introduction
A wide range of technologies are based around the
behaviour and properties of salts either in crystalline form or, more
often, in a molten form. For example the electrical properties of salts
are crucial to their role as electrolytes either as a means of storage
of energy eg in fuel cells or batteries or as a means of refinement or
extraction of metals from ores. Molten salts are also used in high
pressure discharge lamps in order to improve their performance and
their colour rendition. Molten salts can also be used as solvents
facilitating chemical reactions and because of this they are also
potentially corrosive.
The modelling of the processes involved in those
applications requires thermodynamic data for the multicomponent solid
and liquid solutions involved. For this purpose a database has been
developed for the binary, ternary and reciprocal salt systems, which is
used in conjunction with MTDATA in order to make the calculations
required for multicomponent systems. Because all the data are both self
consistent and referred to standard data for the elements they can be
used for calculations in conjunction with other types of phases
including alloys, oxides, gases etc. The database developed covers the
halides, sulphates, carbonates, hydroxides of alkali, some alkaline
earth and other metals.
The KCl-NaCl system
The first system to be examined is a binary
system, in this case the KCl-NaCl system. The ACCESS module is used to
retrieve the data by defining the system and the database where the
data are stored. The thermodynamic data for the liquid phase are
modelled in a way which takes account of the ionic nature of the phase.
The model allows data to be mixed for monovalent ions (eg alkali
metals, halides), divalent ions (eg alkaline earth ions, sulphates,
carbonates) etc. In order to invoke this model it is also necessary to
specify that charged species are required.
WHICH MODULE ? access
ACCESS OPTION ? define system 'KCl,NaCl,Cl/-' source salts !
SEARCHING FOR SYSTEM KCl,NaCl,Cl/-
salts - NPL Salts Database, version 1.0 - 16/3/93
ACCESS OPTION ?
The LIST command can be used to find out the
phases for which data have been retrieved.
ACCESS OPTION ? list system phases !
NUMBER PHASE STATUS MODEL
1 HALITE NORMAL REDLICH-KISTER
2 LIQUID**:1:1 NORMAL SUBLAT. VAR. NO VA
3 MONOCLINIC NORMAL PURE SUBSTANCE
The MONOCLINIC phase is not required for this
particular system and therefore could be removed by using the CLASSIFY
command
ACCESS OPTION ? classify absent phase(mono) !
Finally the SAVE command is used to write the data
into a file for subsequent use. Input of the command RETURN leaves the
ACCESS module and allows entry of the BINARY module in order to
initiate the calculation of the phase diagram.
ACCESS OPTION ? save
SIMPLIFIED MODEL USED FOR PHASE HALITE:1:1
ACCESS OPTION ? return binary
Once in the BINARY module it is necessary to read
in the datafile that has just been created using the ACCESS module.
This is performed by using the DEFINE command. The file had been saved
with the default name DEF.MPI. Unless a filename is specified this file
will be read in.
BINARY OPTION ? define !
The phase diagram of the KCl-NaCl system has a
miscibility gap within the HALITE phase at low temperatures. To
calculate the phase diagram at these low temperatures MTDATA is asked
to look for one using the CLASSIFY command. The LIST command is used to
check that MTDATA has taken note of this.
BINARY OPTION ? classify misc(halite) 1 ! list system ph !
NUMBER PHASE STATUS MODEL
1 HALITE 1 M-G REDLICH-KISTER
2 LIQUID NORMAL SUBLAT. VAR. NO VA
Finally it is necessary to specify the temperature
range for the calculations and to enter the COMPUTE command. In the
first diagram the high temperature region between say 900 K and 1200 K
is examined with a 5 K step size between calculations.
BINARY OPTION ? step t 900 1200 5 ! compute !
A box appears on the screen as a frame for the
calculated phase diagram. The user is invited to use the cursor to
initiate calculation in a two phase field. The phase diagram as
originally calculated is not shown here. Instead the calculated phase
diagram is replotted with experimental information superimposed.
BINARY OPTION ? replot exper 'kclnaclh.exp' go
In a similar way the low temperature phase diagram
can be calculated and the results replotted with experimental data
superimposed.
BINARY OPTION ? step t 500 900 5 ! compute !
BINARY OPTION ? replot exper 'kclnacll.exp' go
The Li2CO3-Na2CO3-K2CO3
system
MTDATA can also be used to calculate phase
diagrams for ternary salt systems. In the following example the Li2CO3-Na2CO3-K2CO3
system is examined which is of potential interest in the design of
materials for use in fuel cells. Firstly the ACCESS module is used to
retrieve and store the data for this system.
WHICH MODULE ? access
ACCESS OPTION ? define system 'K2CO3,Li2CO3,Na2CO3,CO3/-2' sou salts !
SEARCHING FOR SYSTEM K2CO3,Li2CO3,Na2CO3,CO3/-2
salts - NPL Salts Database, version 1.0 - 16/3/93
ACCESS OPTION ? save
Occasionally MTDATA provides helpful warning
messages, here to indicate a possible missing item of data. In this
case it can safely be ignored.
ACCESS OPTION ? return ternary
TERNARY OPTION ?
Having saved the data into a datafile the TERNARY
module can be used to calculate the phase diagram. First it is
necessary to use the DEFINE command to indicate the datafile to be used
- here the default file DEF.MPI. The data are read into the program.
TERNARY OPTION ? define !
The temperature must be specified using the SET
command.
TERNARY OPTION ? set t 700 !
The COMPUTE command initiates the calculation of
the ternary isothermal section
TERNARY OPTION ? comp !
A triangular grid appears on the screen as a frame
for the calculated phase diagram. In an analogous way to the use of the
BINARY module the cursor may then be used to initiate calculation in a
two phase field.
The NaCl-KCl-Na2SO4-K2SO4
system
Many commercial alloys operating at high
temperatures are susceptible to corrosive attack from molten salts.
Typically an alkali metal halide is transported into the vapour phase,
reacts with any sulphur and oxygen species to form sulphates which
deposit on the alloy in the form of a complex solid or molten salt.
This salt mixture may dissolve the protective oxide layers (eg Cr2O3)
on the alloy exposing the material to corrosive attack. One of the key
salt systems responsible for this "hot salt corrosion" is NaCl-KCl-Na2SO4-K2SO4.
These salts form what is called a "reciprocal
system" - there are four pure salts involved linked together by a
chemical equation. Phase diagrams for such a system are similar to
ternary phase diagrams - it is possible to plot isothermal sections but
in this case the four pure components form corners as a square with
each side representing a binary system such as the KCl-NaCl system
shown earlier. In this system the liquid phase alone forms a continuous
series of solutions across the system and must be modelled according to
a model which takes account of the chloride and sulphate anions.
It is of interest to calculate the pseudo binary
section between pure NaCl and K2SO4. To do this it is necessary to use
the "ISOPLETH" application of MTDATA.
WHICH MODULE ? ac
ACCESS OPTION ? def sys 'NaCl,K2SO4,KCl,Cl/-' sou salts !
SEARCHING FOR SYSTEM NaCl,K2SO4,KCl,Cl/-
salts - NPL Salts Database, version 1.0 - 16/3/93
ACCESS OPTION ? save
Isopleths or pseudo binary sections can be
calculated via the APPLICATION module.
ACCESS OPTION ? ret app def !
There is a miscibility gap in the HALITE phase of
the KCl-NaCl system.
APPLICATION OPTION ? cl misc(hal) 1 !
Isopleth calculations are initiated by invoking
the ISOPLETH macro.
APPLICATION OPTION ? 'mt-isopleth.mac'
I S O P L E T H 0 0 1
Isopleth - the calculation of pseudo binary sections through
multicomponent systems
It is now necessary to define certain parameters for the calculations.
Enter composition tolerance (typically 0.0001) ?
: 0.0001
Enter number of temperature steps (perhaps 30) ?
: 100
APPLICATION OPTION ? set "NaCl" 1.0 0.0 0.0 !
APPLICATION OPTION ? set "K2SO4" 0.0 1.0 0.0 !
APPLICATION OPTION ? range "NaCl" "K2SO4" 100 !
APPLICATION OPTION ? range temperature 600 1400 0.1 ! comp !
The calculation then proceeds automatically by
calculating individual phase boundaries for a range of compositions and
temperatures. These appear on the screen as discrete points. Phase
fields may be labelled easily using a graphics cursor which appears
after the calculations have been completed. The diagram may be
replotted in which case the phase boundaries appear as solid lines as
in the next figure.
Reciprocal diagrams
Isothermal sections of reciprocal salt systems can
be calculated using the TERNARY module. Often diagrams for reciprocal
systems are shown with a constant number of cation ions per component
eg Na2Cl2 is used instead
of NaCl. ACCESS allows components to be defined in this way.
WHICH MODULE ? ac
ACCESS OPTION ? def sys 'Na2SO4,Na2Cl2,K2SO4,Cl/-' sou salts !
SEARCHING FOR SYSTEM Na2SO4,Na2Cl2,K2SO4,Cl/-
salts - NPL Salts Database, version 1.0 - 16/3/93
ACCESS OPTION ? save
ACCESS OPTION ? ret ter d !
As before it is necessary to tell MTDATA to look
for a miscibility gap within the HALITE phase. After setting the
temperature the reciprocal isothermal section can be calculated through
use of the compute command specifying that the diagram should be
reciprocal in type rather than using the triangular grid normal for
ternary phase diagrams.
TERNARY OPTION ? clas misc(hal) 1 !
TERNARY OPTION ? set t 900 !
TERNARY OPTION ? comp diag recip !
Diagrams such as the previous isothermal section
can be combined together to form liquidus projections by making use of
an MTDATA metafile. A named metafile is opened and all diagrams
calculated while it is open are saved to it automatically. When the
metafile is closed, the diagrams can be replotted, superimposed, using
the UTILITY module's PLOT command.
TERNARY OPTION ? define !
TERNARY OPTION ? clas misc(hal) 1 !
TERNARY OPTION ? clas misc(hal) 1 !
TERNARY OPTION ? [metafile=liqpro.mta
TERNARY OPTION ? set t 700 ! compute diag recip !
TERNARY OPTION ? set t 750 ! compute diag recip !
TERNARY OPTION ? set t 800 ! compute diag recip !
.....
TERNARY OPTION ? set t 1300 ! compute diag recip !
TERNARY OPTION ? [metafile=none
TERNARY OPTION ? return utility
UTILITY OPTION ? plot file 'liqpro.mta' !
MTDATA metafiles provide an easy way for diagrams
to be combined. Diagrams saved in metafiles can also be edited, before
plotting, for example to add extra annotation.
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