Oxidative leaching of a sulfidic flue dust of former copper shale processing with focus on rhenium

The investigation aims at a hydrometallurgical processing approach for an environmentally hazardous material called “Theisenschlamm”, which is a flue dust of former copper shale processing in Germany. Besides eliminating the negative environmental impact, processing of this material would also be a contribution to a circular economy, since it contains about 16 wt.-% zinc, 14 wt.-% lead, minor amounts of copper and tin, as well as valuable elements of strategic economic importance, such as rhenium, molybdenum and germanium. The mainly sulfidic matrix of the Theisenschlamm was characterised using scanning electron microscopy in combination with QEMSCAN software. Leaching of Theisenschlamm in acidic and alkaline media, as well as the effect of oxidising agents, was studied in order to extract zinc, copper, rhenium, germanium and molybdenum. In both sulphuric acid and sodium hydroxide solutions, the addition of oxidising agents (hydrogen peroxide and ozone) improved metal extraction efficiencies significantly. The leaching system sulphuric acid/hydrogen peroxide was investigated in more detail, with focus on the optimisation of rhenium extraction and its effect on the extraction efficiencies of the other target elements. Response surface methodology was applied with respect to H2SO4 concentration (0.1-1.2 mol/L), H2O2 concentration (0.1-2.8 mol/L) and solid:liquid ratio (40-150 g/L). This study shows that oxidative leaching enables the extraction of zinc, copper, rhenium, germanium and molybdenum from this sulfidic material. In terms of rhenium extraction, a low acid concentration is favourable; however, lowering the acid concentration results in a reduced yield of other target elements (e.g. molybdenum).