Formation of heavy metal bearing phases at a spring affected by the weathering of ore processing residues

Abstract

The Mansfeld region in Sachsen-Anhalt, Central Germany, has a long tradition of mining Kupferschiefer, a marine copper-containing black shale of Permian age. One of the residues from the processing of that low-grade copper ore is ‘Theisen Sludge’, a very fine-grained scrubber dust composed of metal sulphide particles in a matrix of highly viscous hydrocarbons. The Theisen Sludge is considered to be the main source for heavy metal releases into the local environment. The most important discharge point for contaminated water is the ‘Stadtborn Spring’. The extremely high metal and sulphate concentrations in the spring water cause the precipitation of a mineral phase exceptionally rich in heavy metals. This precipitate is the focus of the current investigation. It is shown that a gel with adsorbed metals first is formed from spring water. Then zinc-copper-aluminium hydroxides start to crystallise from the gel at an early stage, forming (Zn,Cu,Al)(O,OH) 2 brucite-type layers, which are positively charged owing to a remarkable A1 content. These precursor layers take up SO 4 2 - for charge compensation and H 2 O into interlayers and begin to form zinc-copper-aluminium hydroxide-sulphates such as Al-rich zincowoodwardite within a few weeks of aging. Further maturation of the precipitate took place in the gaseous environment of atmospheric CO 2 after part of the brook dried up several decades ago. This ‘maturation’ due to the long-term interaction of the precipitate with atmospheric CO 2 is characterised by the transformation of the zinc-copper-aluminium hydroxide-sulphates into zinc hydroxide-carbonates with a residual copper content. The resulting crystalline phase is characterised structurally as disordered hydrozincite. The results described are the first step in a study which aims to optimise the natural precipitation process observed as a part of a remediation strategy for the local surface water system. It also is noticed that the fresh precipitate may be of use as a precursor material for copper/zinc co-precipitated oxide catalysts for industrial hydrogenation processes.