Effect of Prewetting Brines on the Ice-Melting Rate of Salt at Very Cold Temperatures

Abstract

At temperatures close to its eutectic, sodium chloride (NaCl) is shown to have substantial ice-melting capacity, but the ice-melting rate is very slow. The effects of magnesium chloride (MgCl₂), NaCl, and mixed MgCl₂-NaCl brines on the ice-melting rate of prewetted NaCl rock salt at temperatures close to the NaCl eutectic were measured. The solid salt component of a 50:50 mixture of salt and 26.0% MgCl₂ brine melted ice 7.1 times faster than dry salt after 3 h, 3.5 times faster after 7 h, and 1.7 times faster after 24 h at an average temperature of −4.48F (−20.2°C). The solid salt component of a 50:50 mixture of salt and 23.3% NaCl brine melted ice 2.9 times faster than dry salt after 3 h, 2.0 times faster after 7 h, and 1.6 times faster after 24 h at the same temperatures. Ice-melting rates with mixed MgCl₂-NaCl prewetting brines were intermediate between those of the straight NaCl and MgCl₂ prewetting brines. Above a ratio of prewetting brine-to-salt high enough so that the ice was completely covered with brine, there was no significant increase in the ice-melting rate with increased prewetting brine application rate. Preliminary evidence suggests that a contribution to the rate enhancement caused by MgCl₂ may be enhanced diffusion of NaCl from the dissolving salt crystals in the MgCl₂ brines.