The present study handled an economic and clean route for the synthesis of pure Mo and Fe–Mo alloys through utilization of waste materials such as spent acid containing Mo and mill scale removed from the surface of steel slabs. The Mo oxide, MoO
, was successfully synthesized from the spent acid by ammonia gas neutralization method and reduced under hydrogen atmosphere to produce a high-purity Mo powder. The optimum condition for the spherical shape and uniform particle size of Mo powder was determined. In addition, the recycled MoO
from the spent acid was mixed with mill scale as an iron oxide source to produce Fe–Mo alloy by the hydrogen reduction. Based on the thermogravimetric analysis, the reduction rate of the mixed MoO
and mill scale was measured at 700–1100 °C. The Arrhenius equation with the approved mathematical formulations for the heterogeneous gas–solid reaction was applied to calculate the activation energy (Ea) values and determine the rate controlling mechanisms.