Weathering of metalliferous alum shales

A considerable amount of S, Fe, Mo, V, and other elements are leached out by rainwater as soluble compounds from the weathering products of alum shales. Laboratory experiments show that thermal decomposition of the organic matter of alum shales after spontaneous ignition, oxidation or exposure to the bacteria Thiobacillus ferrooxidans substantially increases the release of metals from the shale. The most pronounced separation (up to 45%) of heavy metals by extraction with water (at 1:5 s/w ratio) at room temperature (18 to 20 °C) was achieved in case of alum shale roasted at 400 to 800°.