An incongruent heat-of-fusion system—CaCl2·6H2O—Made congruent through modification of the chemical composition of the system

Abstract

Incongruent melting with phase separation is a frequent cause of failure of heat-of-fusion systems. In devices based on CaCl₂·6H₂O, formation of a tetrahydrate poses such problems. This paper describes means of making the incongruent system congruent, i.e. ensuring that the tetrahydrate never is the stable species, through chemically modifying the system. Thus, addition of SrCl₂·6H₂O to a CaCl₂H₂O system causes the solubility of CaCl₂·6H₂O to decrease and CaCl₂·4H₂O to increase. The reasons for the different effects on the hexahydrate and the tetrahydrate are related to decreased activities in the solid state through formation of a solid solution in the former case and to the decreased activities in the liquid state in the latter case (i.e. the phenomenon of freezing point depression). If the addition of SrCl₂·6H₂O is sufficiently large (about 2 wt per cent), the melting point maximum for CaCl₂·6H₂O coincides with the peritectic point for equilibrium between the hexahydrate, the tetrahydrate, and the solution. With technical grade materials, tetrahydrate formation is a more severe problem as they contain impurities (KCl and NaCl) which have an opposite influence on the solubilities of the CaCl₂ hydrates as described above for SrCl₂·6H₂O. Addition of Ca(OH)₂ is shown to be an effective tool in suppressing tetrahydrate formation in these cases. It acts through neutralizing the excess of chloride and through formation of the phase CaCl₂·CaO·2H₂O.