An empirical rate law for gypsum powder dissolution

摘要

The dissolution rates, normalized by surface area, of two different gypsum powders in aqueous solutions were measured by tracking changes in solution composition as a function of time in a continuously stirred batch reactor. The powders were classified into different size ranges to observe the influence of particle coarseness on dissolution kinetics. Changes in solid surface area during the experiment were modeled as a function of mass loss by measuring the specific surface areas of the powders before and after the experiments. Contrary to the implicit assumption in several previous studies, the total powder surface area increases, typically by a factor of two, prior to decreasing with ongoing dissolution. This phenomenon therefore has a significant influence, up to 50%, on the surface area-normalized rate. The initial solution composition was set to one of several prescribed saturation states so that an empirical rate law could be inferred over a wide range of saturations. The rate coefficient for both types of gypsum depended on size, being lower for coarser particles. Other than the difference in rate coefficients, the dissolution rate of both sources of gypsum can be described well by the same mathematical function of solution saturation, at least to within the uncertainty in the measurements. Data from previous studies of gypsum dissolution appear to follow approximately the same dependence on particle size as observed in this study.