Mineralogy and environmental stability of vanadium-rich slags from the historical processing of Zn-Pb-V ores at Berg Aukas (Namibia)

Vanadium (V) is one of the key technologically critical elements. The slags produced by the historical mining and processing of Zn–Pb–V ores in the Waelz kiln at Berg Aukas (northern Namibia) can be potentially used as a secondary resource of V. A combination of mineralogical methods, bulk chemistry, leaching tests and speciation-solubility modeling was used to understand the binding of the major contaminants (Zn, Pb, V) in the solid phase and their potential release under the changing environmental conditions. The average concentrations of the metal(loid) contaminants in the slags are 3.78 wt% Zn, 3370 mg/kg Pb, 5880 mg/kg V, 767 mg/kg Cu, 578 mg/kg As and 92 mg/kg Sb. The mineralogy is dominated by high-temperature silicates (clinopyroxene, melilite, olivine-family phases) and Zn-bearing phases (willemite, zincite). All the primary silicates and oxides are Zn-rich, but vanadium is mainly concentrated in clinopyroxene (up to 5 wt% V2O3). Metallic Fe inclusions, formed under highly reducing conditions in the kiln, are highly weathered. Secondary Fe(III) (hydr)oxides, corresponding to the main weathering products in the slag, efficiently sequester the metal(loid)s (mainly As and Sb). The EU regulatory leaching tests indicated that the release of the metal(loid) contaminants is quite low at the natural pH (deionized water extract: 8.5–10.4) obtained by extraction in the deionized water and only Sb in all the slag samples exceeds the EU limits for the landfilling of inert waste. The pH-static leaching tests revealed up to 5 orders of magnitude higher release of Pb and Zn under acidic conditions (up to 38% and 63% of their total concentration, respectively), compared to the natural pH. In contrast, V exhibits relatively flat pH-dependent leaching patterns with only <1.6% of the total V leached. Using the slag re-processing costs by acidic (bio)leaching and the current metal prices, the recovery of V, being the most important critical metal in the Berg Aukas slags, seems to be non-economical (Ettler et al., 2020, Applied Geochemistry, DOI: 10.1016/j.apgeochem.2019.104473). This study was supported by the Czech Science Foundation project (GAČR 19-18513S).