Stabilization and phase transformation of CuFe₂O₄ sintered from simulated copper-laden sludge

The feasibility of stabilizing copper-laden sludge by high-temperature CuFe₂O₄ ferritization process is investigated with different sintering temperature, and the prolonged leaching test. The thermal behavior, structural morphology, phase composition, and phase transformation of the stabilized sludge were investigated by using thermal gravimetry-differential scanning calorimetry, scanning electron microscopy and X-ray diffraction. The leaching behavior of the stabilized sludge under acidic environment was evaluated by modified Toxicity Characteristic Leaching Procedure (TCLP). The results indicated that CuFe₂O₄ could be effectively formed at around 800°C by the iron oxide precursor with a 3h of short sintering. The transformation was discovered on crystallographic spinel structures: the low-temperature (800-900°C) tetragonal phase (t-CuFe₂O₄) and the high-temperature (~1000°C) cubic phase (c-CuFe₂O₄). At higher temperatures (~1100°C), the formation of cuprous ferrite delafossite phase (CuFeO₂) from the dissociation of CuFe₂O₄ was also noticed. Both CuFe₂O₄ spinel and CuFeO₂ delafossite phase have a better intrinsic resistance to acidic environment when compared to that of CuO phase by the modified TCLP test. The sintering strategy designed for copper-laden sludge is proven to be beneficial in stabilizing copper.