Kinetics of celestite conversion to acidic strontium oxalate hydrate in aqueous solution of oxalic acid

Abstract

Conversion of SrSO4 to acidic strontium oxalate hydrate (H[Sr(C2O4)(1.5)(H2O)]) in aqueous H2C2O4 solutions proceeds as a consecutive reaction. In the first step of the consecutive reaction, SrSO4 reacts with H2C2O4 and pseudomorphic conversion to SrC2O4 center dot H2O occurs. In the second step, SrC2O4 center dot H-2 reacts with H2C2O4 to form H[Sr(C2O4)(1.5)(H2O)]center dot Sr(HC2O4)(C2O4)(0.5)center dot H2O crystallizes during cooling of the reaction mixture to room temperature if the solution reaches the saturation concentration of H[Sr(C2O4)(1.5)(H2O)]. The aims of this study are the derivation of reaction rate equations and the determination of the kinetic parameters such as pre-exponential factor, apparent activation energy and order of H2C2O4 concentration for each reaction step. Fractional conversions of SrSO4 were calculated using the quantitative amounts of dissolved S and Sr. It was determined that the reaction rate increased at the initial time of reaction by increasing the temperature using solutions with approximately same H2C2O4 concentrations. The reaction extends very slowly after a certain time in solutions with low H2C2O4 concentration and ends by the formation of a protective layer of SrC2O4 center dot H2O around the surfaces of solid particles. Fractional conversion of SrSO4 is increased by increasing concentration of H2C2O4 at constant temperature. Kinetic model equations were derived using shrinking core model for each step.