Unveiling drying kinetics and crack evolution during vacuum drying of washed MSWI fly ash

Washed municipal solid waste incineration (MSWI) fly ash typically leaves the washing unit as a cohesive, high-moisture cake, and a subsequent drying step is often required to enable safe handling, transport, and downstream stabilization/solidification. This study applies vacuum drying to washed fly ash using a static vacuum dryer. We investigate how heating temperature (80 °C–140 °C), vacuum level (gauge pressure ranging from 0 to −0.08 MPa), and fly ash layer thickness (3.4–13.6 mm) affect drying behavior, kinetics, and mechanisms. Temperature and layer thickness are the dominant factors controlling the drying rate, while the influence of vacuum level is moderate but non-negligible. A mild-to-moderate vacuum accelerated drying by lowering the equilibrium boiling temperature and sustaining a large vapor-pressure driving force, whereas excessively strong vacuum tended to promote early surface stiffening and crust formation, and thereby limited internal moisture transport. From the fitted kinetics, an apparent effective diffusivity and an Arrhenius-type apparent activation energy were estimated; Ea decreased from 44 kJ/mol in the early stage to 15 kJ/mol in the late stage. Time-lapse imaging revealed substantial shrinkage and crack development: higher temperature and greater thickness promoted main cracking, while pressure primarily regulated fine cracks (milder vacuum produced denser fine cracks; deeper vacuum yielded fewer, larger cracks). These findings provide practical insights for optimizing vacuum drying of washed incineration fly ash, aiding in its safe reuse or disposal and contributing to resource utilization and environmental protection.