Evaluating CRM potential and environmental impact of Sardinian mining and metallurgical waste: the cases of Su Suergiu and Corti Rosas antimony mines (Southeastern Sardinia)

The supply of Critical and Strategic Raw Materials (CRM, SRM) is a key challenge for the EU. To reduce dependency on third countries, the Critical Raw Materials Act (2024) sets targets for domestic sourcing and recycling from extractive waste, which represent the second-largest source of waste in EU, with about 622 million tons produced annually (EUROSTAT, 2023). Much of this waste comes from abandoned mining sites, where it was stockpiled for years, posing significant environmental hazards. Yet, these wastes may contain significant amount of SRM and CRM, which could now be considered for recovery. In Italy, Sardinia is a key area of interest due to its rich mining legacy and the considerable volumes of mining waste left behind after the closure of the last active mines, which hold significant CRM potential. Specifically, this study focuses on evaluating the potential of mining waste in the Gerrei area (SE Sardinia), where about 1 Mm3 of mining and metallurgical residues were disposed in dumps (Naitza et al., 2019). The study investigates the Su Suergiu and Corti Rosas mines, which represented the largest industrial complex for antimony in Italy until the 1980s. Our research has confirmed that the metallurgical slags at Su Suergiu are significantly enriched in Sb (up to 9.75 wt%) and could be viable for its recovery. In contrast, flotation tailings from Corti Rosas generally show lower Sb concentrations, ranging between 0.1 and 5 wt%, still of interest for further investigation of its recoverability. In addition to Sb, these wastes contain other valuable metals in potentially useful concentrations, such as Zn (up to 4.47 wt%), W, and Au (up to some g/t according to Garbarino et al., 2003). Finally waste rock dumps show metal contents too low to envisage any possibility of recovery. A major environmental concern at both Su Suergiu and Corti Rosas is the presence of alkaline drainage (pH 8–11), mainly originating from Na-rich slags and from flotation tailings. This atypical basic leachate, unlike classic Acid Mine Drainage (AMD), promotes the mobilization of other potentially toxic elements such as Sb and, As, whereas the only AMD yielding sample largely mobilizes Zn and Cu. Contaminated runoff flows into local streams (Rio Sessini and Rio Ciurixeda at Su Suergiu) and then discharges into the Flumendosa River. This river, the largest in southeastern Sardinia, also receives input from Corti Rosas and other minor mining sites in the Gerrei district, resulting in a contamination plume extending over ~20 km to the Tyrrhenian Sea (Fancello et al., 2023, and references therein). Our results showed that the slag samples (Su Suergiu) are potentially the most polluting waste, with tailings at Corti Rosas mine being the second, while contribution from waste rocks is negligible. The recovery of these metals is important and could be feasible even in subeconomic conditions, considering how strongly it would reduce the environmental pollution in this area.