Cyanide-Free Thiosulfate Leaching of Gold from Pelletized Pyrite Concentrate Calcine Using Ashless Polymer Binders

This study investigates a cyanide-free approach to gold recovery from pyrite concentrate produced by the Almalyk Mining and Metallurgical Complex. The proposed process combines pelletization, oxidative roasting, and ammoniacal thiosulfate leaching. A pyrite concentrate containing 4.0–4.5 g t⁻¹ Au was pelletized into 3–5 mm granules using SK-K and SK-D ashless polymers or kaolin as binders at concentrations of 1–3 wt.%. The pellets were dried at 105 °C and roasted at 600 °C for 60 min to remove sulfur, decompose the pyrite structure, and form a porous calcine suitable for subsequent leaching. Gold leaching was conducted for 2 h at 300 rpm using 0.10–0.15 mol L⁻¹ Na₂S₂O₃ and 0.01–0.02 mol L⁻¹ CuSO₄ at pH 9–10. The formation of the [Au(S₂O₃)₂]³⁻ complex was evaluated by UV–Vis spectrophotometry in the 200–350 nm range. The ashless polymer binders produced substantially stronger pellets than kaolin. At 600 °C, pellets containing 3 wt.% SK-K exhibited the highest compressive strength of 1.65 MPa, compared with 1.48 MPa for 3 wt.% SK-D and 1.28 MPa for 3 wt.% kaolin. The leachate obtained from SK-K-bound pellets showed the most intense absorption bands in the 260–310 nm region, including a pronounced maximum at 305 nm, indicating enhanced formation and stability of the gold–thiosulfate complex. SK-D also provided a strong spectral response, whereas the kaolin-containing sample showed lower absorbance. The results demonstrate that ashless polymer binders improve pellet mechanical stability, prevent mineral dilution, and promote gold transfer into thiosulfate solution.