Synthesis and properties of SrFe₁₂O₁₉ obtained by solid waste recycling of oily cold rolling mill sludge

The aim of this study was to develop a new approach for the preparation of environmentally friendly, high-value products from oily cold rolling mill (CRM) sludge. Utilizing oily CRM sludge as a source of iron, strontium hexaferrite (SrFe₁₂O₁₉) powders were prepared by multi-step processes involving acid leaching, chemical conversion treatment, and synthesis by a citrate precursor. The influences of citric acid dosage and the pH of the sol system on the formation, crystallite size, and magnetic properties of the obtained SrFe₁₂O₁₉ powders were investigated. High saturation magnetization (74.8 mA·m²·g^-1) and intrinsic coercivities (614.46 mT) were achieved for pH 7.0 of the sol system, for which the molar ratio of citric acid dosage to the total dosage of Fe³⁺ and Sr²⁺ was 1.5. This study presents a new approach to utilizing oily CRM sludge, and even refractory iron-containing solid waste.