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 degreesC, 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 similar to10(-4) M, consistent with a high purity gypsum starting material) and a rate constant ksimilar to0.12 h(-1) at 95 degreesC. Variations in the observed half-life for transformation found between the solid-state (similar to65 h) and the solution (similar to77 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. (C) 2004 Published by Elsevier B.V.
