<p>In this study, different concentrations of Fe₃O₄ were introduced to modify chromium-containing sludge, and conventional solid-phase sintering was employed to concentrate chromium (Cr) element into the chromite spinel phase, thereby immobilizing the chromium in the sludge. A series of synthesized samples were characterized using X-ray diffraction, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, as well as dielectric and magnetic property measurements. The results showed that all samples contained three phases: spinel, phosphate, and amorphous. When the Fe/Cr ratio in the spinel phase exceeded 1.9/1.1, the samples exhibited significant magnetism. Subsequently, the effects of microwave irradiation on the liberation behavior of the spinel phase, as well as on the microstructure and mechanical properties of the ceramic samples, were systematically investigated. Scanning electron microscopy (SEM) analysis revealed that microcracks appeared in the microwave-treated ceramic samples, which was attributed to the selective heating characteristics of microwaves and the differential thermal expansion among different phases. Moreover, as microwave irradiation time increased, the crack density and the degree of sample damage also increased. The product obtained by magnetic separation of the sample irradiated with microwaves for 7&#xa0;min had a spinel content of 89.19 wt%. The magnetic separation experiments indicated that the separation efficiency was improved due to the intergranular cracks generated by microwave treatment.</p> Graphical Abstract <p></p>