The kinetics of the precipitation of gypsum, CaSO4·2H2O, over a wide range of reactant concentrations

• Published in: Reaction kinetics, mechanisms and catalysis mechanisms and catalysis, 2020-10, Vol.131 (1), p.75-88
• Main Authors: Ziegenheim, Sz, Peintler, G., Pálinkó, I., Sipos, P.
• Format: Article
• Language: English
• Subjects: Catalysis; Chemistry; Chemistry and Materials Science; Industrial Chemistry/Chemical Engineering; Physical Chemistry
• ISSN: 1878-5190; 1878-5204
• DOI: 10.1007/s11144-020-01838-0

In this work, the precipitation kinetics of gypsum was studied over a wide range of degree of supersaturation at 25 °C in the reaction Na 2 SO 4 aq + CaCl 2 aq + 2 H 2 O → 2 NaCl aq + CaSO 4 · 2 H 2 O S with the aim of constructing a comprehensive kinetic model for CaSO 4 ·2H 2 O (s) formation that is valid from the lowest (0.04 M) to the highest (0.20 M) feasible initial reactant concentration. To monitor the variation of reactant concentrations during the precipitation reaction, conductometry was employed. For reasonably slow reactions (where the establishment of the equilibrium potential on the indicator electrode was possible), the measurements were supplemented by a Ca-ion-selective electrode. The structure and morphology of the precipitating solids was characterized by XRD and SEM. The induction period was found to decrease about two orders of magnitude with the increasing reactant concentration. It was experimentally established that the influence of the so-called wall effect is of secondary importance. Using the data collected, a kinetic model have been suggested that can describe the entire precipitation process of gypsum simultaneously, incorporating nucleation and crystal growth, in a wide concentration range. Our calculations strongly suggest that the inclusion of the CaSO 4(aq) ion pair is necessary for the appropriate kinetic description of gypsum precipitation.