Hydrothermal Synthesis of Hydroxyapatite-Rich Calcium Phosphate from Converter Steel Slag via Sequential Water–Acetic Acid Leaching

Converter steel slag contains abundant calcium-bearing phases but remains underutilised because of its heterogeneous composition and the potential co-release of undesirable elements. In this study, a hydroxyapatite-rich calcium-phosphate powder was prepared from converter steel slag through sequential water–acetic acid leaching followed by phosphate precipitation and hydrothermal treatment. The slag was first leached with deionised water at a liquid-to-solid ratio of 50 mL·g−1 for 30 min at 25 °C. The residue was subsequently leached with 1 mol·L−1 acetic acid at 65 °C for 1 h. The combined leachates were reacted with phosphoric acid, adjusted to pH 11 using sodium hydroxide, and hydrothermally treated at 120 °C for 48 h. X-ray diffraction indicated that hydroxyapatite was the dominant identifiable crystalline phase, accompanied by a minor β-tricalcium phosphate phase. Fourier-transform infrared spectroscopy confirmed phosphate- and hydroxyl-related functional groups. Scanning electron microscopy revealed predominantly needle-like particles with lengths of approximately 1–12 μm. Inductively coupled plasma optical emission spectrometry gave calcium and phosphorus concentrations of 183.96 and 82.33 mg·L−1, respectively, corresponding to an apparent Ca/P molar ratio of approximately 1.73. In a 24 h extract assay using human bone marrow-derived mesenchymal stem cells, the relative cell viability was 103.74% ± 1.82% at 1% extract and decreased to 65.22% ± 4.81% at 5% extract and 5.31% ± 0.73% at 100% extract. The 1% extract group did not differ significantly from the control. The 5–100% extract groups showed significantly lower viability than the control group, with p < 0.01. These results demonstrate the feasibility of converting calcium-bearing components of converter steel slag into a hydroxyapatite-rich calcium-phosphate powder at laboratory scale. Further work is required to establish calcium mass balance, quantitative phase composition, trace-element distribution, process reproducibility, and long-term biological performance.