INFLUENCE OF CRYSTALLIZATION CONDITIONS ON THE CRYSTAL GROWTH MECHANISM AND CRYSTAL GROWTH RATE OF YTTRIUM OXALATE

摘要

In the seed crystallization of yttrium oxalate in metastable supersaturation the modification of the growth mechanism by crystallization coditions of agitation speed and additive was observed. From the desupersaturation curve the crystal growth rate was expressed with a power law of supersaturation (RT=K_g△C~p). Without on additive the growth rate coefficient, Kg, increased with increasing the agitation speed but the growth rate order, p, was slightly dependent on the agitation speed. When an additive of sodium tripolyphosphate was used, the desupersaturation in the solution was much retarded due to inhibition of the crystal growth and the growth rate order was varied up to about 2.6 with increasing the additive concentration. By using a two-step growth model for discription of the crystal growth mechanism it was obtained that the surface integraiton rate was proportional to a square of supersaturation and the mass transfer coefficient increased with increasing the agitation and 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 but the mass transfer coefficient was almost invariant. The shift of the rate-determining step by the additive was examined with the effectiveness factor. The effect of the additive on the surface integration was depicted with Langmuir isotherm.