QUATERNARY EUTECTIC SALT MIXTURE CONTAINING LIF

MELANGE EUTECTIQUE SALIN DE TYPE QUATERNAIRE CONTENANT DU LIF

English Abstract

ABSTRACT

Heat storage material comprising an eutectic salt
mixture wh1ch consists of about 44 mol.% of L1F, about 40
mol.% of KF, about 12 mol.% of NaF and about 4 mol.% of MgF2.


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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PRO-
PERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Salt mixture which contains LiF and has a eutectic
melting point between 400°C and 500°C, characterized in that
it contains about 44 mol.% of LiF, about 40 mol.% of KF, about
12 mol.% of NaF and about 4 mol.% of MgF2.
2. Method of producing a salt mixture containing LiF
as claimed in Claim 1, characterized in that a salt mixture
consisting of 44 mol.% ? 2 mol.% of LiF, 12 mol.% ? 1 mol.% of
NaF, 40 mol.% ? 2 mol.% of KF and 4 mol.% ? MgF2 is prepared
and is repeatedly melted and cooled until a clear melt which
melts at 450°C is obtained which is decanted from any residue
which has not melted at this temperature.


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Description

PHN 7011



` ~0~
The invention relates to materials which have
a melting point between 400C and 500C, maximum
melting heat and heat capacity and which do not attack
usual container materials, for example nickel-chromium
S steel.
Materials of this type may be used as heat storage
materials, for example in solar power plants, in storage
heaters and the like, as a heat transfer material, for
example in nuclear reactors, and in salt baths for
heat treatment.
A ternary eutectic mixture is known which melts
at 454C and consists of 46.5 mol% of LiF, 42.0 molt ~
of KF and 11.5 mol.% of NaF.
In particular in power stations, for example
solar power stations, such a material may be used in
very large quantities. The comparatively high price
and comparatively rarity of lithiumfluoride may become
important factors under such conditions.
The present invention provides a salt mixture
which has an eutectic melting point between 400C and
500C and is characterized in that it contains about 44
mol % of LiF, about 40 mol.% of KF, about 12 mol.% of
NaF and 4 mol. % of MgF2. The melting point of this
eutectic mixture is about 449C. The heat of fusion
is about 167 cal/g and hence exceeds that of the eutec-
tic mixture of LiF, KF and NaF (heat of fusion about


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plrl\T 7011
21~ 1974

~ 046261

163 cal/g) by ~out 2.5 ~. Cons,equently, with equal storage
capacity in the form of heat of fusion the quantity of
salt mixture according to the invention may be smaller
than that of the kno~-n mixture. In addition, the mixture
according to the in~ention contains less I,iF, ~-hich means
additional saving in expensive Li~.
A 4uaternary salt mixture consisting of 44.9
mol.~O of LiF, 40.6 mol.~ of KF, 11.5 mol.~ of ~aF and
3 mol.~0 of CaF2, melting point 444GC, has already been
proposed as a heat-storage material (French Patent Specific-
ation 1,52,4,997). This mixture contains a greater quantity
of lithiwnfluoride than the mixture according to the in-
vention and has a heat of fusion of about 1~9 cal/g, ~-h,ch
is smaller than that of the kno~m miYture of LiF, ~ and
hra~ and that o thc m-Yture accord~ng to the in~e~tion~
' The novel salt mixture can be obtained in the
follo~.ring manner. A salt mixture is prepared wllich
contains
' 44 mol.~ + 2 mol. ~ of LiF,
12 mol.~o + 1 mol. ~ ~aF,
40 mol.~ + 2 mol. Gp ~F and
Ll mol.~ nol.~, of MgF2.
The mixt;lrc is repeatedl~- melted an~ slowly
cooled until meltin~ at 4500C results iIl a clear ~elt.
The melt is decanted from any residlle ~hich at this
tempe~ure h~ls not ~lelted, a~d then is cooled. The COID-
positi~n of the melt then corresponds to the eutectic

Pa-~ ~ o 1 1
24-4-1974

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mixture having substantially the following composition:
44 mol.~ of LiF, 40 mol. ~ of KF, 12 mol.% of NaF and 4 mol.%
of MgF2
The salt mixture may be used as a heat storage
material and as a heat transfer material. The mixture is
chemically stable, non-combustible and doe~ not attack nickel-
chromium steels; in operation it does not produce vapour,
which is of importance for use in closed containers.-