Abstract: The paper describes the application of hydrothermal sulfidation and flotation for the metal recovery from the sludge which is produced after the metallurgical waste water treatment. Experiment is carried out to investigate the influencing factors of hydrothermal sulfidation by orthogonal experimentation. The result shows that temperature is the most significant factor on the sulfidation of Zn, followed by solid -to -liquid ratio, reaction time and reagent dosage of sulfur. The optimum operating process parameters are established as follows: reaction time (4 h); temperature (200 °C); solid-to-liquid ratio(3∶1);the reagent dosage of sulfur (18 %). Under these experimental conditions, the sulfidation extent of zinc is greater than 90 %. The subsequent flotation investigates the effects of particle size, depressant dosage (CMC), temperature and pH value on the flotabilities of synthetic ZnS. The results indicate that temperature is the major factor to influence the flotation. Under the optimal conditions (particle size: 38 μm, temperature: 60 ℃, CMC: 200 g/t and pH value: 8), the grade of zinc concentrate can reach 28.8 % with recovery efficiency of 55.16 % in one roughing stage .