Gypsum–anhydrite transformation in hot acidic manganese sulfate solution. A comparative kinetic study employing several analytical methods

Transformation of calcium sulfate dihydrate (gypsum) into anhydrous calcium sulfate (anhydrite) proceeds slowly in a manganese sulfate/sulfuric acid medium (36 g Mn2+/kg of solution and 36 g H2SO4/kg of solution) at 95 °C, conditions pertinent to some industrial processes, without any apparent intermediate species. The kinetics of the transformation have been probed by following the change in mole fraction of gypsum and/or anhydrite versus time in both solution (dissolved [Ca], by ICP-AES) and solid-state (XRD and TGA). The kinetics fit an autocatalytic process, with a very small initial [product]o (typically ∼10−4 M, consistent with a high purity gypsum starting material) and a rate constant k∼0.12 h−1 at 95 °C. Variations in the observed half-life for transformation found between the solid-state (∼65 h) and the solution (∼77 h) measurements may reflect non-equilibrium solution behaviour associated with slow crystallization of anhydrite, consistent with a dissolution–precipitation mechanism. Addition of an anhydrite seed at commencement of the reaction diminishes the induction period and accelerates the transformation, as expected for an autocatalytic process.