Neutralization sludge (NS), a calcium-based heavy metal waste, was produced in large quantities during
wastewater treatment using lime milk neutralization in the nonferrous metal industry. During hydrothermal
sulfidation and flotation of NS, the phase transition of calcium compounds might play a crucial
role in the subsequent flotation for low metal recovery. During hydrothermal sulfidation, calcium minerals
in NS were converted and further developed into bulk CaSO4 crystals with many nano-ZnS crystals
adhering on its surface. Specifically, the conversion of CaCO3 into CaSO4 could accelerate the sulfidation
of NS. However, the presence of CaSO4 could slow the growth rate of ZnS crystals, which imposes a negative
effect on subsequent flotation. CaSO4 crystals with irregular shapes and incomplete crystallization
combined with nano-ZnS crystals were frequently observed in concentrates after flotation, whereas
CaSO4 crystals of regular shape and high crystallinity were mainly discovered in tailings, indicating that
modifying the crystalline nature and texture of CaSO4 crystals is a feasible method for separating ZnS and
CaSO4 and for increasing metal recovery. Furthermore, the adsorption of collectors on CaSO4 was hardly
detected, indicating that CaSO4 barely competed with ZnS for collectors during flotation.