Recycling red mud with raw coal to low-cost Fe/C/ceramic composite for efficient microwave absorption

To solve the urgent issue of electromagnetic (EM) wave radiation pollution and promote the resource utilisation of red mud (RM, a solid waste), Fe/C/ceramic composite EM wave-absorbing materials were constructed by recycling RM with raw coal (RC) through simple mechanical mixing and then carbothermal reduction between Fe₂O₃ in RM and carbon component in RC. It was found that the calcined temperature of 900 °C can be considered the optimal formation temperature for Fe. In addition, a tuneable EM wave absorption performance could be attained by regulating the mass ratio of RC to RM (denoted as <i>M</i>_RC:<i>M</i>_RM). When the <i>M</i>_RC:<i>M</i>_RM value reaches to 0.4:1 and 0.5:1, the composites exhibit more favourable performance. The composite with <i>M</i>_RC:<i>M</i>_RM of 0.4:1 showed the minimum reflection loss (<i>RL</i>_min) of -41.6 dB, accompanied by an effective absorption bandwidth (<i>EAB)</i> of 3.2 GHz when the simulating thickness was 2 mm. The composite with <i>M</i>_RC:<i>M</i>_RM of 0.5:1 possessed the maximum <i>EAB</i> of 4.2 GHz with the <i>RL</i>_min of -25.3 dB at a thickness of 1.5 mm. The satisfactory performance profits from good impedance matching and strong intrinsic attenuation capability. The former can be attributed to the regulatable EM parameters of the multicomponent system; the latter is mainly credited to the strong dielectric loss arising from the medium graphitised carbon, highly crystalline Fe, and plentiful defects and interfaces in the composites. This work not only provides a valid path to realise the economical preparation of microwave absorbents but also achieves the rational disposal of RM.