Hydrothermal sulfidation-flotation is considered to be a feasible means for the heavy metal recovery from neutralization sludge (NS) generated by the lime-neutralization process in our previous studies. However, the as-generated sulfides prefer to aggregate on the surface of CaSO4 during hydrothermal sulfidation, which causes difficulties in the sulfides recovery via flotation. In this study, the hydration of CaSO4 was proposed to promote the separation of sulfides (mainly ZnS) in sulfidized NS. When hydration was performed on pure CaSO4, the rod-shaped CaSO4 with an average length of 10 pm was converted into CaSO4 center dot 2H(2)O with an increased average length of 100-400 mu m. Based on this, hydration was introduced into the sulfidized NS, and the CaSO4 in the sulfidized NS was converted into bulk CaSO4 center dot 2H(2)O crystals. Sieving experiments toward the hydration product show that a total of 90.66% of the Zn was concentrated in a -48 mu m sieving product. To further improve the zinc grade of the -48 mu m sieving product, a flotation test was conducted, during which a concentrate with Zn grade of 37.11% was obtained and Zn recovery reached 66.87%. In contrast, when flotation was directly performed on sulfidized NS, the Zn grade in concentrates and Zn recovery were only 21.96% and 37.43%, respectively. Therefore, the hydration of CaSO4 in sulfidized NS can effectively promote the separation and recovery of ZnS. (C) 2015 Elsevier B.V. All rights reserved.