Mineralogical Immobilization and Resource Utilization of Chromium from Chromium-Containing Sludge via Narrow-Particle-Size-Fraction (Fe, Mg)1–xO Solid-Solution Magnetization

The low grade, high activity, and high toxicity of chromium in chromium-containing sludge present significant challenges for its immobilization and recycling. To effectively stabilize and recover chromium from chromium-containing sludge, this study presents a novel method referred to as “narrow-particle-size- fraction (Fe, Mg)1–xO solid-solution magnetization”. By prescreening the appropriate particle size fraction of (Fe, Mg)1–xO, this process leverages the thermodynamic and kinetic advantages offered by high temperatures to convert the toxic and highly mobile chromium component into a nontoxic, structurally stable, and strongly magnetic (Fe, Mg)3–xCrxO4 (0 ≤ x ≤ 2) solid-solution during roasting. The (Fe, Mg)3–xCrxO4 phase is then isolated using weak magnetic separation. Taken the chromium-containing sludge with a Cr2O3 content of 5.94 wt % as the raw material, after optimizing the (Fe, Mg)1–xO particle size fraction and roasting conditions, the chromium recovery rate in the magnetic concentrate exceeds 92%, while the grade of Cr2O3 increases to 12.47%. Various analyses confirm that the (Fe, Mg)3–xCrxO4 solid-solution is an effective magnetic carrier for chromium in chromium-containing sludge. This study provides valuable insight for the mineralogical immobilization and resource utilization of chromium-containing solid waste.