Microwave-assisted carbothermic reduction of red mud using palm kernel shell charcoal: A circular economy approach for sustainable iron recovery

Red mud, a byproduct of bauxite processing, presents a significant environmental challenge with 4.6 billion tons currently stored globally and 4.3 million tons generated annually in Indonesia. Simultaneously, palm kernel shells from crude palm oil (CPO) processing represent an underutilized biomass resource of 13.2–14.3 million tons annually. This work presents a microwave-assisted reduction (MAR) process for the simultaneous valorization of both waste streams by recovering iron from red mud using palm kernel shell charcoal (PKSC) as a renewable reductant. Microwave carbothermic reduction at 2.45 GHz was employed using briquettes of red mud and PKSC, with systematic investigation of microwave power (500–900 W), reduction time (2.5–30 min), and carbon-to-oxygen (C/O) molar ratio (0.8–2.2). Characterization was conducted via X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), and metallization analysis. Maximum iron metallization of 94.3% was achieved under optimized conditions (800 W, 30 min, C/O = 1.4) with a peak temperature of 1214 °C, without the need for chemical additives. This result represents a 50% reduction in processing time compared to conventional electric furnace methods (60 min at 1400 °C achieving 97.2% metallization) at a 186 °C lower temperature. Thermodynamic analysis identified critical temperature thresholds for sequential oxide reduction pathways (Fe 2 O 3 → Fe 3 O 4 → FeO → Fe). The microwave-selective heating of iron oxides provides a competitive alternative to conventional external heating methods, particularly due to its volumetric and rapid heating characteristics. This process demonstrates the technical and economic viability of integrating two major Indonesian industrial waste streams into a value-added product through energy-efficient pyrometallurgical processing, with implications for establishing domestic iron raw materials supply and advancing circular economy objectives. • Dual waste valorization of red mud and palm kernel shell supports circular economy. • Biomass-derived charcoal enables sustainable and low-impact iron recovery. • Microwave-assisted reduction improves energy efficiency of iron extraction. • Recovered iron shows potential to substitute imported raw materials for steelmaking. • The process integrates environmental remediation with industrial resource recovery.