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

Results are presented of a series of experiments in which hyper-eutect ic aqueous solutions of NH4Cl and NH4Br contaminated by small amounts of CuSO4 are cooled from below. The experiments with NH4Cl indicate th at beyond a contamination of 0.3 wt%, the well-known chimney structure that is associated with solidification from a pure aqueous solution o f NH4Cl disappears. Subsidiary experiments indicate that at approximat ely the same level of contamination, the branched dendritic morphology that is usually associated with crystalline NH4Cl is suppressed. It i s suggested that the macroscopic chimney-driven compositional convecti on 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.