Exploring Convective Drying Behavior of Hydroxide Sludges Through Micro-Drying Systems

The drying of hydroxide sludge is a critical step in its valorization process in drinking water treatment plants (WWTPs), due to the high energy requirements associated with this operation. This study investigates the convective drying behavior of hydroxide sludge using a convective micro-dryer, with air heated to temperatures between 70 °C and 110 °C, velocities ranging from 1 m/s to 3 m/s, and constant absolute humidity of 0.005 kg of water per kg of dry air. The process was continuously monitored through X-ray microtomography, allowing the nondestructive observation of external surface texture evolution, shrinkage, and crack formation. A significant shrinkage, with a volume reduction ranging from 30% to 45%, was observed as the moisture content decreased. The experimental data were used to develop a characteristic drying curve specific to hydroxide sludge, which remained consistent across different operational conditions. The results showed that increasing air temperature and velocity enhanced the drying flux and reduced drying time, while higher air humidity produced the opposite effect. Additionally, the crack formation observed towards the end of the drying process was associated with internal moisture transfer limitations. Effective diffusivity increased with air temperature, highlighting the significant impact of temperature on the activation energy of the drying process. These findings provide valuable insights for optimizing the energy efficiency of sludge-drying operations.