Preliminary Study on In Situ Immobilization of Pb, Cd, and Zn in Flotation Tailings and Metallurgical Slags Using Phosphate, Cement, and Iron-Based Additives

Flotation tailings and metallurgical slags from mining often contain toxic Pb, Cd, and Zn. In this study, we evaluated the in situ immobilization of Pb, Cd, and Zn in a Pb–Zn flotation tailing and a smelting slag by adding representative amendments: phosphate-based (ammonium phosphate, phosphoric acid, glassy fertiliser), cementitious (Portland cement), and iron-based (bog iron ore) materials at 1–10% (w/w). Treated samples underwent EPA-TCLP and pH-dependent leaching tests (pH 3–10), with Pb, Cd, and Zn measured by atomic absorption spectroscopy. The untreated tailing leached hazardous Pb (~60 mg/L) and elevated levels of Cd (~0.7 mg/L) and Zn (~53 mg/L), whereas the untreated slag leached negligible metal concentrations. All amendments reduced metal release in a dose-dependent manner. Phosphate amendments were most effective (e.g., 10% H3PO4 cut tailing Pb by 80%, Cd by 60%, and Zn by 30%), while cement and iron additions had much weaker effects. Solid-phase XRD and SEM-EDS analyses indicated the formation of stable calcium–phosphate minerals on sulfide surfaces after phosphate treatment. These findings suggest that low-cost phosphate additives (~5–10%) can substantially immobilize Pb, Cd, and Zn in such wastes. However, under strongly acidic conditions (pH < 3), some remobilization occurred, highlighting the need for further validation. This work provides practical guidance for waste managers on selecting in situ stabilization strategies for Pb–Zn mine wastes.