Studies on nucleation and crystal growth kinetics of uranium(IV) oxalate

Abstract

Abstract An improved apparatus is used for nucleation measurements according to Nielsen’s method. A new method is proposed to calculate the dilution ratio N of the reaction solution during nucleation rate determination. With the rule, when the initial apparent supersaturation ratio S′ = f(N) in the dilution tank is controlled from 1.2 to 2.7, crystal nucleus dissolving and secondary nucleation can be avoided satisfactorily. Experiments are realized by varying the supersaturation ratio from 26.0 to 297.5 and temperature from 30 °C to 50 °C. Uranium(IV) oxalate is precipitated by mixing equal volumes of tetravalent uranium nitrate and oxalic acid solution. The experimental results show that the nucleation rate of uranium(IV) oxalate in the supersaturation range as show above is characterized by the primary homogeneous mechanism and can be expressed by the equation R N = A N exp ( − E a / R T ) exp [ − B / ( ln  S ) 2 ] , ${R_N} = {A_N}{\rm{exp}}( - {E_a}/RT){\rm{exp}}[ - B/{({\rm{ln }}S)^2}],$ where AN  = 1.9 × 1027 m−3s−1, Ea  = 71.2 kJ mol−1, and B = 34.3. The crystal growth rate can be expressed by the equation G ( t ) = k g exp( − E ′ a / R T )( c − c eq ) g , $G(t) = {k_g}{\rm{exp(}} - {E^{\prime}_a}/RT{\rm{)(}}c - {c_{{\rm{eq}}}}{{\rm{)}}^g},$ where kg  = 7.1 × 105 (mol/L)−0.98 (m/s), E ′ a = 33.1  kJ mol − 1 , ${E^{\prime}_a} = 33.1 \ {\rm{ kJ \ mo}}{{\rm{l}}^{ - 1}},$ and g = 0.98. The results indicate that the crystal growth of tetravalent uranium(IV) oxalate is controlled by the BCF model.