Why is molybdenite roasted in an excess of air?

Molybdenite (molybdenum disulfide, ${\text{MoS}}_2$) is roasted in an excess of air to facilitate its conversion into technical molybdic oxide (${\text{MoO}}_3$).12

This process is essential because approximately 97 percent of ${\text{MoS}}_2$ must be transformed into this oxide form to reach its commercial destination. During roasting, the excess air provides the necessary oxygen to drive the exothermic reaction that replaces the sulfur in the mineral with oxygen. This reaction is carefully controlled by adjusting the oxygen supply and using cooling mechanisms to ensure the temperature remains below 650 °C (1,200 °F), preventing the ${\text{MoO}}_3$ from sublimating. The process continues until the sulfur content of the resulting calcines is reduced to below 0.1 percent. Once purified, this technical molybdic oxide serves as the primary material for producing molybdenum metal and various other molybdenum-based compounds.123