Solubility of Gold in FeO_x-SiO₂-Al₂O₃ Slags at 1300°C

Solubility of gold in copper-free alumina iron silicate slags in equilibrium with iron alumina spinel was studied using equilibration-quenching technique under controlled p(O₂) gas atmosphere at 1300°C. The gold concentration in slag was analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), which allowed elimination of the mechanical metal dispersion from the slag analyses. Thus, the data represent purely the chemically dissolved gold, and the dissolution in slag was shown to occur as AuO_0.5 species at high oxygen partial pressures ≥10⁻⁷ atm. Contrary to earlier observations in (basaltic) iron-lean high-silica melts, the results indicate that gold solubility at low oxygen partial pressures, <10⁻⁷ atm, was no more decreasing but maintained at a minimum (log₁₀L^Au/s(Au) = 5.6–5.7). Even at the highest partial pressure of oxygen (=10⁻⁵ atm), the distribution coefficient was above log₁₀L^Au/s(Au) = 5.2, which suggests that mechanical dispersion containing gold causes by far the major source of gold loss in the copper and nickel matte smelting as well as in the secondary copper (‘black copper’) smelting operations.