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Encyclopedia of Physical Science and Technology EN008H-970 June 29, 2001 16:46

664 Liquid Alkali Metals

300 C, attack becomes evident in 3 days and in several the need to maintain perfect protection from the atmo-
◦
hours, respectively. The discoloration of glass is due partly sphere. However, since the liquid is a metal it is possi-
to the elemental silicon produced by the reduction process ble to take advantage of the electromagnetic effect, and
all the liquid alkali metals can be stirred readily by ro-
4M + SiO 2 = 2M 2 O + Si
tating a permanent magnet outside the containing ves-
and partly by penetration of alkali metal atoms into the sel. This technique has now been generally superseded
glass. Attack on glass by liquid potassium, rubidium, and by the electromagnetic pump, which employs the prin-
cesium is somewhat slower than that by sodium at similar ciple that if a conductor carrying a current is placed at
temperatures. However, because of their lower melting right angles to an applied magnetic field, there will be a
points, these metals can be handled as liquids at lower mechanical force on the conductor in a direction perpen-
temperatures.Asampleofcesiumthathadbeenkeptunder dicular to both the magnetic field and the direction of the
argon in a glass vessel at room temperature gave no sign current. Small pumps can be incorporated into steel con-
of attack after 4 years. tainers for laboratory work, at temperatures up to 500 C.
◦
In practice, the liquid alkali metals are handled almost Extensive use of electromagnetic pumps has also been
entirely in equipment constructed from transition metals. made for circulating the alkali metal in sodium-cooled
The refractory metals (e.g., titanium, zirconium, vana- fast reactors; these are large-capacity pumps requiring cor-
dium, chromium, molybdenum, and tungsten) are all use- respondingly high currents and powerful electromagnets
ful, but they are expensive and sometimes difficult to fabri- and are now being superseded by mechanical (centrifugal)
cate and are reserved for parts of the industrial plant where pumps.
their special properties are needed. The element that finds
by far the widest use is iron, and by alloying iron with
chromium and nickel it is possible to devise steels that are IV. PURIFICATION
appropriate for the various experimental conditions and
temperatures. All the main components of a steel have The solubility in the liquid alkali metals of those metals
some slight solubility in liquid sodium, and where many thatcanbeusedascontainersisquitesmallandcanusually
tons of sodium are circulating in steel containers this can be measured in parts per million. The solubility of most of
be significant. Some relevant solubility values are quoted the nonmetals is also small. Nevertheless, it is these small
in Table II. With liquid sodium, steels can be devised in concentrations of metals and nonmetals that are responsi-
which this solubility problem is minimized, but the chem- ble for many of the problems that arise in the industrial use
istry of liquid lithium–steel interactions is different in two of the liquid alkali metals. For this reason, much attention
important respects. First, since nitrogen has a consider- has been paid to methods of purification and analysis of
able solubility in liquid lithium (1.7 mol% at 420 C), it the liquid metals. Heavy metal impurities are introduced
◦
replaces oxygen as the element mostly responsible for cor- as a result of corrosion of structural metals either dur-
rosion of steels by liquid lithium. Second, the solubility ing production or storage. The main metallic impurity in
of iron and the alloying elements (especially nickel) are sodium is calcium, which results from the production of
much higher in liquid lithium than in liquid sodium, and sodium by electrolysis of NaCl–CaCl 2 mixtures. The main
even for short-term laboratory experiments it is desirable nonmetallic impurities are hydrogen and oxygen (from
to contain liquid lithium in pure iron vessels. contact with the atmosphere and moisture), carbon (from
structural steels) and nitrogen, which is only important
C. Stirring in the case of lithium. Some solubility values are given
in Table III. They illustrate the influence of temperature
Stirring the liquid alkali metals inside suitable contain-
and the very high solubility of oxygen in liquid rubidium
ers is more difficult than with most liquids because of
and cesium, which is exceptional. There are three main
methods of purification, involving filtration, gettering, or
TABLE II Approximate Solubilities of Some distillation.
Metals at 650 C in Liquid Lithium and Liquid
◦
Sodium
Solubility (ppm by weight) A. Filtration
Solute
metal In liquid lithium In liquid sodium The liquid metals can be readily filtered by passage
through sintered plates made from glass or steel. Sim-
Ni 1000 1.7
ple filtration at constant temperature will merely remove
Cr 15 0.12
suspended foreign matter, but advantage is taken of the
Fe 16 1.0
fact that solubilities in the alkali metals decrease with