The post-Variscan hydrothermal vein system at Silius (Southeastern Sardinia, Italy) is known mostly for its fluorite-barite-galena mineralization, which has been exploited until early 2006 and is now in maintenance. Distinct fluorspar amounts still exist in the mine, and are evaluated at 2 million tonnes of measured resource at 34.5% CaF 2 (727,495 tonnes fluorspar) and 3.2% Pb (67,724 Pb tonnes in galena). Gangue carbonates of the veins, which consist of calcite and ferroan dolomite, contain the REE- minerals synchysite-(Ce) and xenotime. To check the effective amounts of REE in the Silius orebody, representative samples of the carbonate gangue have been collected from several locations in the mine. ICP-MS analyses have been carried out in the samples with the aim to obtain the bulk chemical composition of the carbonates. The concentrations throughout the mine range between 462 and 2,071 ppm (951 ppm on average), mainly consisting of LREE. The average volume of the carbonate gangue still in place, which would be extracted together with fluorite and galena, is currently considered to be around 532,000 tonnes, to which more 750,000 tonnes of carbonates discarded in the old waste dumps may be added. The corresponding total REE amounts currently occurring in the Silius gangue may sum up to 1,220 tonnes pure REE. The discovery of these amounts of REE in the Silius fluorspar mine may open interesting perspectives for the exploration of subeconomic REE concentrations in this type of deposits, where REE could be recovered as by-product of the fluorite exploitation.
The present research represents an approach toward the recycling of extractive waste inspired by circular economy and sustainability that is developed in accordance with Goal 12 of the United Nations 2030 Agenda for Sustainable Development Goals. A new procedure for the recovery of REEs from fluorite–barite–galena ores with calcite gangue from the Silius mine (Sardinia, Italy) is presented. The considered samples are waste materials of Silius mineralization, collected in the old processing plant of Assemini (near Cagliari). In this orebody, REE minerals consist of prevailing synchysite (a REE-bearing fluorocarbonate) and subordinate xenotime-Y (a Y-bearing phosphate). REE fluorocarbonates are extracted using 50% K2CO3 as the leaching solution, at 100 °C. Using a solution (mL)/sample (g) ratio of 25, about 10% of the total REE content of the considered sample is extracted within 1 h. At the laboratory scale, such alkaline leaching of REE from the waste materials allows the recovery of the CO2 produced as K2CO3 from concentrated KOH, in accordance with a circular flow. Further work is ongoing to scale up the process into a pilot plant, to prove that the method developed within this research can be economically feasible, socially suitable, and environmentally respectful.
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