The crystallisation of alkaline‐earth metal molybdates and tungstates from lithium chloride melts by continuous cooling. Nucleation mechanism and kinetics and induction periods

Abstract

Calcium, strontium and barium molybdate (and tungstate) solutions in lithium chloridc melts were crystallised in alumina and in platinum crucibles ; saturated solutions were cooled from initial temperatures 700' to 800 "C down to room temperature a t cooling rates 40" to 200 "C hr-l. The nucleation and early crystal growth were investigated by chemical and differential thermal analysis and by optical microscopy studies. Crystallisation occurred through heterogeneous nucleation a t low supersaturations. Heterogeneous nuclei formed slowly onto metal aluminate (and platinate) particles within the solution during induction periods from <0.2 to 14 hr. The main growth surge then started and few new nuclei were formed.The nucleation probably terminated a t times just after the times for maximum rate of formation of nuclei. Then, a t any temperature, the induction periods (t) varied inversely with cooling rate and with the rate (R,) of development of excess solute concentration according to the relation, -where?= t, = Ilk, a t R, = 1 gfg hr-l, y = 0.9 to 1.3.The parameters k , were related to the rate constants (k,) for the heterogeneous nucleation. These constants in turn probably dependend on the free energy for formation of critical heterogeneous nuclei and thence on some nucleator vs solute surface energy 'wetting' function. The k , and k, values a t any temperature decreased in the order Ca 3 Sr > Ba salt; metal WO, > MOO,: