Numerical Investigation of a Continuous Microwave Rotary Kiln for the Reduction of Iron-Rich Byproducts Generated During Steelmaking

Microwave processing offers a sustainable alternative to conventional fossil-fuel-based high-temperature treatments of steelmaking residues by enabling volumetric and energy-efficient heating. This work develops the first fully coupled electromagnetic–thermal–fluid–chemical Multiphysics model of a continuous microwave rotary kiln for the simultaneous reduction of iron oxides and zinc volatilization from a mixture of Basic Oxygen Furnace (BOF) dust and Blast Furnace (BF) sludge. The model integrates time-harmonic Maxwell’s equations (915 MHz), porous media transport, shrinking-core kinetics for seven heterogeneous reactions, and energy/momentum conservation in a rotating quartz tube reactor. Validation against experimental thermogravimetric and literature thermal data confirmed high accuracy. Simulations achieved microwave coupling efficiencies above 97% with localized heating inside the resonant cavity. At a fixed mass flow rate of 5 kg/h, the degree of reduction reached up to 70% with peak temperatures of 1333–1429 K. Fixed-reduction analysis (70% target) identified a maximum sustainable throughput of 2.37 kg/h, limited by quartz tube thermal constraints. Results reveal a trade-off between thermal/energy efficiency (favored at higher throughput) and specific direct CO2 emissions (lower at reduced throughput due to moderated Boudouard reaction). This modeling framework provides valuable guidelines to design energy-efficient, low-emission microwave-assisted metallurgical processes.