Investigation of Metastable Zone Width and Primary Nucleation Kinetics in Oxalic Acid Crystallization Using Acetonitrile–Water Mixed Solvent Systems

To optimize industrial crystallization of oxalic acid, the metastable zone width (MSZW), molecular dynamics, and primary nucleation behavior of oxalic acid in acetonitrile–water systems were examined. The study quantitatively assessed the influence of saturation temperature (303.15–323.15 K), solvent composition (33–66 wt % acetonitrile), cooling rate (3–21 K/h), and agitation speed (200–500 rpm) on the metastable zone width in acetonitrile–water solutions via laser methods. Experimental findings indicated that MSZW increased with lower agitation speeds and saturation temperatures, and with higher cooling rates. Molecular dynamics simulations confirmed that increased acetonitrile mass fraction inhibits oxalic acid diffusion, thereby expanding the MSZW. This suppression arises from reduced solvent polarity and elevated viscosity at higher acetonitrile ratios, which diminish diffusion coefficients and impede both nucleation and crystal growth, consistent with experimental MSZW broadening. Nucleation kinetic parameters were obtained using a self-consistent Nývlt-like equation and a modified Sangwal model, with the calculated nucleation order (m) ranging from 2.6 to 3.8, suggesting a shift from instantaneous to progressive nucleation across the experimental temperature range. This comprehensive analysis offers valuable theoretical insights for enhancing the industrial crystallization of oxalic acid, particularly in mixed solvent systems.