Extensive weathering of zinc smelting slag in a heap in Upper Silesia (Poland): Potential environmental risks posed by mechanical disturbance of slag deposits

- Slag heap after Zn-Pb smelting was disturbed during removal.
- The interior comprises fine grained material dominated by secondary phases.
- Interior of the slag heap can be more intensely weathered than the outside parts.
- The weathering is probably facilitated by long-term disposal.
- The weathered material can pose environmental risk due to high metal contents.

Slags from base metal smelting are often deposited for over 100 years with no barriers between the heap and the surrounding environment. Observations at the external surfaces of the slag heaps may not reflect the state of the slag material inside the slag heap. A slag heap from Zn-Pb ore smelting (Świętochłowice, Upper Silesia, Poland) was disturbed during recent slag removal and the freshly uncovered surfaces are examined in this study. The material forming the interior of the slag heap is fine-grained (up to 5 cm) and strongly weathered (called the weathered slag zone) in contrast to the large slag boulders on the surface of the slag heap (up to 2 m), which are only slightly weathered. The weathered slag zone is composed of gypsum and hematite plus a mixture of primary and other secondary phases. The weathered material as a whole is chemically more homogenous than unweathered slags and has lower Si and higher Fe, Pb (up to 3 wt.%) and Cd (up to 560 mg/kg) concentrations. SEM images show that primary slag phases are porous and disintegrated. The examined surfaces are 3-4 m high and 10-30 m wide suggesting that such slag weathering may have occurred in more extensive parts of the slag heap. Weathering affects slag types with different phase compositions and is therefore controlled by specific conditions occurring within the heap, not by the type of slag. These conditions are so far not well defined, but may include prolonged slag deposition, slow water transfer within the heap and interaction of slags with acid rains common in the Upper Silesia region.The weathered slag zone, when confined within the slag heap, acts as a sink for many potentially toxic elements, decreasing their release to the environment. However, this may change dramatically when the zone is uncovered and its deposit conditions change. Potential risks encompass release of potentially toxic elements from the heap, transport of fine grained particles to the surrounding environment and release of elements when the material is deposited in nearby soils or waters, as well as when it is reused as a construction material.