Effect of Residual Carbon on Vanadium and Scandium Leaching from Thermally and SteamThermally Treated Metalliferous Oil Shale

Abstract

The leachability of vanadium and scandium was compared for two technologically distinct products derived from metalliferous oil shale of the Kyzylkum region, Uzbekistan: thermal-treatment ash produced without a dedicated decarbonization step and flotation tailings from steam-thermally treated shale after removal of the carbonaceous fraction. Two existing sulfuric-acid leaching datasets were evaluated under closely comparable conditions. At a liquid-to-solid ratio of 3 mL g⁻¹, 75–80 °C, and approximately 60 min, average vanadium recovery was 35.8% from thermal ash and 74.7% from flotation tailings, whereas scandium recovery was 31.5% and 78.8%, respectively. Maximum recoveries within the investigated parameter ranges were 56.7% versus 90.0% for V and 54.8% versus 98.6% for Sc. The thermal ash retained residual organic matter, reflected by a loss on ignition of 11.48%, while the flotation product had undergone targeted decarbonization. The markedly lower recovery from the carbon-bearing ash is therefore consistent with parasitic re-sorption of dissolved metal species by residual carbon, a mechanism previously observed for related material. For the decarbonized tailings, a 4×4 factorial dataset yielded statistically significant effects of both sulfuric-acid concentration and leaching time: R² = 0.864 for V and 0.965 for Sc, with p < 0.05 for both factors. The results identify decarbonization as a quantitatively important process variable and support its direct experimental evaluation as a pretreatment step for improving V and Sc recovery from thermally treated oil-shale residues.