Dehydration of gypsum waste to recyclable anhydrite using a nano-film reservoir under ambient conditions

Abstract

The accumulation of gypsum waste (GW), such as flue gas desulfurisation gypsum and phosphogypsum, which remains in regulated stacks occupying considerable land resources, is causing significant environment problems worldwide. A promising strategy for treatment and recycling of GW is to convert GW into higher-valued anhydrite, but the traditional energy-intensive calcination method significantly increases the cost of recovery. Here, we report the first solid-like dehydration of gypsum to produce anhydrite at room temperature. Specifically, a 39 nm thick sulfuric acid film (NSF) was fabricated to extract water of crystallization from gypsum and simultaneously enable a phase transformation from gypsum to anhydrite via recrystallization. Using density functional theory calculations, we obtained the activity (0.025) of crystalline water in gypsum, which is higher than the corresponding value in NSF, so that the dehydration can occur spontaneously. Using our developed kinetic model, we attributed the acceleration of this transformation to the lower nucleation potential (56 kJ mol⁻¹) of anhydrite and the more efficient transfer of the mass (several orders of magnitude higher than that for heating gypsum powders) required for growth. This model allows prediction of nucleation and growth contributions from kinetic data for any given recrystallization system. This work not only demonstrates a new type of recrystallization reaction driven by a nano-film reservoir, which differs from bulk solution–solid and gas–solid reactions but also provides a promising strategy for energy-efficient recycling of anhydrite from gypsum waste.