Peculiarities of crystallization process in the solution under microgravity (ERDENET experiment)

Abstract

The paper presents the results of the studies of the mass crystallization process, related to the Soviet-Mongolian ERDENET experiment on copper sulphate crystallization from its water solution, conducted aboard the Salyut-6 orbital station. The a priori analysis provided a complete set of equations that determine the kinetics of the crystallization from the solution (melt) in case of systematic and fluctuating microaccelerations typical of orbital stations. The system of equations was solved analytically for the case of crystallization in the presence of a small amount of microseeds. It is shown that fluctuating microaccelerations cause specific inertial random walk of crystals that are not fixed. In the Stokes region of motion velocities the inertial random walk becomes more intensive in the process of crystal growth. The inertial random walk brings about a redistribution of crystals over the bulk of the crystallizer and to solution (melt) stirring. Experimental studies of crystals obtained in space and in a series of ground simulation tests have revealed the following typical features of the crystallization process. In microgravity conditions, heterogeneous nucleation occurred in the bulk of the solution and on the chamber walls; the result was two orders of magnitude more crystals than on Earth. In case of microgravity and the absence of Marangoni convection all crystals were growing in the conditions of full stirring. Typical of space-grown crystals is a very pronounced complex face morphosculpture of crystals. Specular smooth surface is characteristic of Earth-grown crystals.