Recovery of iron from iron tailings by co-pyrolysis with coal gangue: process optimization and economic evaluation

Iron tailings and coal gangue are voluminous solid waste byproducts. However, traditional disposal methods and conventional reduction roasting often are hindered by high costs and a reliance on external fossil fuels as reductants. To overcome these challenges, this study introduces a novel co-pyrolysis process utilizing coal gangue as the sole reductant source for recovering iron from tailings. Key process parameters, including co-pyrolysis temperature (600 - 900 °C), co-pyrolysis time (20 - 60 min), coal gangue ratio (10 - 50 wt%), and magnetic separation intensity (60 - 140 mT), were optimized. In optimal conditions (800 °C, 30 min, 20% wt%, 100 mT), an iron concentrate with 59.10% total Fe (TFe) and 75.4% recovery rate was obtained. Thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM) revealed that the pyrolysis of coal gangue generated reducing gases (CO, H₂, CH₄) and fixed carbon. These reductants were capable of converting the weakly magnetic hematite (Fe₂O₃) to the strongly magnetic magnetite (Fe₃O₄), facilitating efficient low-intensity magnetic separation. An economic evaluation of a production scale of 250,000 t/y showed a net cost of 86.79 USD/t after accounting for byproduct revenue. Importantly, aiming for a zero-waste process, the residual waste was successfully transformed into high-strength non-fired bricks, providing a technically viable and economically superior closed-loop solution compared to existing single-stream utilization methods.