How does roasting zinc sulfide concentrate remove sulfur?

Roasting zinc sulfide concentrate removes sulfur through a highly exothermic oxidation reaction. During this process, the sulfur is replaced by oxygen, converting the metal sulfides into oxides. The removed sulfur is released in the form of sulfur dioxide gas.12

The specific method of roasting depends on the intended extraction process:

• Fluidized-bed roasters: Used for electrolytic production, this method suspends finely divided particles of concentrate in a rising stream of air. This process is thermally efficient and can reduce the sulfur content to less than 0.5 percent, producing a high-strength sulfur dioxide gas that is typically captured for use in a sulfuric acid plant.1
• Sintering: Used when the concentrate grade is lower or lead content is high, this process involves mixing fine concentrates with crushed returned sinter. The mixture is placed on a moving grate and fired in an updraft of air, resulting in a strong, lumpy oxidized feed suitable for a blast furnace.1