Caesium Chloride - Occurrence and Production

Occurrence and Production

Caesium chloride occurs naturally as an impurity in the halide minerals carnallite (KMgCl₃·6H₂O with up to 0.002% CsCl), sylvite (KCl) and kainite (MgSO₄·KCl·3H₂O), and in mineral waters. For example, the water of Bad Dürkheim spa, which was used in isolation of caesium, contained about 0.17 mg/L of CsCl. None of these minerals are commercially important.

On industrial scale, CsCl is produced from the mineral pollucite, which is powdered and treated with hydrochloric acid at elevated temperature. The extract is treated with antimony chloride, iodine monochloride, or cerium(IV) chloride to give the poorly soluble double salt, e.g.:

CsCl + SbCl₃ → CsSbCl₄

Treatment of the double with hydrogen sulfide gives CsCl:

2 CsSbCl₄ + 3 H₂S → 2 CsCl + Sb₂S₃ + 8 HCl

High-purity CsCl is also produced from recrystallized Cs (and Cs) by thermal decomposition:

Cs → 2 CsCl + ICl

Only about 20 tonnes of caesium compounds, with a major contribution from CsCl, were being produced annually around the 1970s and 2000s worldwide. Caesium chloride enriched with caesium-137 for radiation therapy applications is produced at a single facility Mayak in the Ural Region of Russia and is sold internationally through a UK dealer. The salt is synthesized at 200 °C because of its hygroscopic nature and sealed in a thimble-shaped steel container which is then enclosed into another steel casing. The sealing is required to protect the salt from moisture.