SOLIDIFICATION OF NH4CL AND NH4BR FROM AQUEOUS-SOLUTIONS CONTAMINATED BY CUSO4 - THE EXTINCTION OF CHIMNEYS

Results are presented of a series of experiments in which hyper-eutectic aqueous solutions of NH4Cl and NH4Br contaminated by small amounts of CuSO4 are cooled from below. The experiments with NH4Cl indicate that beyond a contamination of 0.3 wt%, the well-known chimney structure that is associated with solidification from a pure aqueous solution of NH4Cl disappears. Subsidiary experiments indicate that at approximately the same level of contamination, the branched dendritic morphology that is usually associated with crystalline NH4Cl is suppressed. It is suggested that the macroscopic chimney-driven compositional convection that results from cooling hyper-eutectic aqueous solutions of NH4Cl is rather sensitive to the microscopic details of the crystal growth. Solidification experiments with pure hyper-eutectic aqueous solutions of NH4Br display a similar chimney-driven convective pattern; however, this is effectively unchanged even in the presence of contaminations of up to 1 Wt% of CuSO4.