Modification of crystal growth mechanism of yttrium oxalate in metastable solution

In the seeded crystallization of yttrium oxalate from a metastable supersaturated solution, the growth mechanism was observed to become modified by variations in agitation and additive concentration. From the desupersaturation curve, the crystal growth rate was estimated and described using a power law of supersaturation (R-T = k(gDelta)C(p))Without an additive in the metastable solution, the overall growth rate coefficient, K-g, increased with an increase in the agitation speed while the growth rate order, p, was almost independent of the agitation speed. When the additive sodium tripolyphosphate was used to inhibit crystal growth, the desupersaturation in the solution was substantially retarded and the growth rate order varied up to about 2.5 with an increase in the additive concentration. Using a two-step growth model, the kinetic parameter dependencies of the crystal growth of yttrium oxalate, such as the mass transfer coefficient, surface integration order, and surface integration coefficient, were estimated in relation to the agitation speed and additive concentration. The mass transfer coefficient increased with an increase in the agitation, whereas the surface integration coefficient was independent of the agitation speed. However, as the additive concentration in the solution increased, the surface integration coefficient was reduced while the mass transfer coefficient remained almost invariant. The change in the rate-determining step by the additive was examined using an effectiveness factor. It was shown that the effect of the additive on the surface integration followed a Langmuir type isotherm. (C) 2002 Elsevier Science B.V. All rights reserved.