Characterization, permittivities and drying kinetics during microwave dewatering of deep-sea polymetallic nodules

• Polymetallic nodules exhibit high permittivities and strong microwave absorption across temperature and frequency ranges. • Permittivity evolution aligns with TGA-identified water-release stages, enabling clear interpretation of drying kinetics. • Microwave dewatering shows rapid, power- and mass-dependent drying kinetics, with secondary peaks tied to strongly bound/tunnel water. • Microwave heating significantly reduces energy use and CO₂ emissions compared with conventional thermal drying. Deep-sea polymetallic nodules are receiving increasing attention as a potential source of several valuable metals including nickel, cobalt, copper, manganese, and iron. These nodules form over millions of years through precipitation processes on the ocean floor and they contain substantial amounts of both free and chemically bonded water. Once retrieved, they are transported to land based extraction facilities that typically employ pyrometallurgical processes to recover the valuable metals. Transferring and removing this water substantially increases processing costs, highlighting the need for energy efficient dewatering technologies that can utilize clean electricity. In the present study, microwave dewatering of polymetallic nodules was investigated at the laboratory scale. The nodules were characterized by X-ray fluorescence (XRF) and X-ray diffraction (XRD), and their real and imaginary permittivities were measured as functions of temperature and frequency using the cavity perturbation technique. The permittivity values were relatively high, confirming that the nodules are excellent microwave absorbers. Thermogravimetric analysis established a strong correlation between permittivity variation and water removal. Microwave dewatering tests showed high drying rates, which increased with input power and sample mass but decreased with increasing water load. Compared with conventional thermal drying, microwave heating demonstrated significantly lower energy consumption and potential reductions in associated CO 2 emissions due to its rapid volumetric heating and efficient coupling with the material. The results demonstrate that microwave heating is an efficient and controllable method for dewatering deep-sea polymetallic nodules prior to metal extraction, and establish a clear link between permittivity and dewatering performance.