Roasting Kinetics of Molybdenite Concentrates

Roasting of molybdenite concentrate is an important stage in the production of commercial molybdenum trioxide. The oxidation kinetics and mechanism of molybdenum sulfide concentrates from two plants were investigated at temperatures from 444 °C to 640 °C. The rate of reaction was determined for thin layer samples in a muffle furnace at controlled temperatures, oxygen excess, and removal of sulfur dioxide. For both concentrates, the rate of reaction is fairly slow at temperatures below 500 °C and remains fairly constant from 540 to 600 °C. It was observed that the concentrate from the first plant presents a tendency for sintering or glazing at temperatures above 620 °C, whereas the same phenomenon was observed at 20 or 30 °C higher temperature for the second concentrate. A kinetic model of two reactions, involving the formation of MoO2 as intermediate oxide for the production of MoO3 is proposed and activation energies of 23.0 and 20.8 kJ/mol for the first and second reaction were calculated for temperatures between 520 °C to 600 °C. These low values of activation energies indicate that the overall rate of oxidation is controlled by mass transfer, particularly in a porous layer of oxides. Therefore, the renewal of the surface by mixing and motion of the concentrate layer is of great importance. Sublimation of MoO3 becomes significant for temperatures above 650 °C.