Waste, landfill sites and mining sludge pollute the environment. In addition to pollutants, they also contain valuable raw materials such as zinc and iron. Scientists at the Institute of Technical Chemistry have joined forces with regional companies to recover and reprocess these materials. They have found a pilot project for this practically on their doorstep in the form of the Roter Graben. Built in the 17th century, the artificial ditch still drains the Freiberg mining district and flows into the Freiberg Mulde. The problem: precipitates containing iron can lead to pollution of surface and groundwater, especially during floods. In addition, the ditch currently only carries five centimetres of water, the rest is contaminated sludge, a total of 13,000 cubic metres.

This sludge is now to be pumped into a so-called chamber filter press on site and dewatered. Membranes are used to remove the heavy metals contained in the filtrate, such as arsenic, lead and cadmium. These are separated and concentrated. If the purity of the metals obtained in this way can then be verified, they can be used in the semiconductor industry, the solar industry or in the production of metal alloys, for example. After the filtering process, clean water remains, which flows back into the Freiberg trough. Recyclable materials such as zinc and iron are extracted from the sludge. The remaining solid residue is also recycled, from which so-called geopolymer building materials are produced. Their structure is modelled on natural minerals.

The resulting material hardens quickly, is pressure-resistant, temperature-resistant, dimensionally stable and non-combustible. All of this makes it an alternative to cement as a raw material for the construction industry - for refurbishment in mining, for wastewater and sewer construction, but also as a heat-resistant material. Initial tests in the laboratory and in a research container have confirmed the success of these recovery processes. The next step is to set up a demonstration plant directly at the Roter Graben under real conditions. This development not only makes it possible to produce building materials cost-effectively, the reutilisation of secondary raw materials also avoids energy-intensive processing of primary raw materials. Landfill costs are eliminated and the carbon footprint is improved.