Which furnace is commonly used to roast molybdenite concentrate?

The conversion of molybdenite concentrate into technical molybdic oxide is almost universally performed using Nichols-Herreshoff-type multiple-hearth furnaces.1

In this process, the concentrate is fed into the top of the furnace, where it moves downward against an upward current of heated air and gases. Each hearth within the furnace features rotating arms equipped with rabble blades that rake the material, allowing it to drop to the next level. This setup facilitates the conversion of molybdenum disulfide ($Mo{S}_2$) into molybdenum trioxide ($Mo{O}_3$) through an exothermic reaction.1

Operators carefully control the temperature to ensure it remains below 650 °C (1,200 °F) to prevent the sublimation of the resulting oxide from its solid state. The roasting is considered complete once the sulfur content of the material is reduced to below 0.1 percent.1