Reprocessing historical tailings for possible remediation and recovery of critical metals and minerals : The Yxsjöberg case

Sammanfattning: With increasing consumption of primary mineral resources, the generation of mining wastes has also increased thus requiring the mining industry to address the growing environmental concerns. Waste rock and tailings are potential secondary sources of critical raw materials currently in short supply in the European Union, but also a source for environmental issues like acid mine drainage. Therefore, reprocessing of such tailings to recover critical metals and minerals is not only a supply risk-reducing measure but also an approach to remediation.Sweden, as a country with an active metal mining industry, is exploring historical tailings repositories for possible remediation and recovery of critical metals and minerals. The challenge is that information about the tailings material and the repositories is frequently limited. In this thesis, a conceptual framework has been defined and employed to systematically generate relevant information and knowledge about the Smaltjärnen tailings repository of the Yxsjöberg historical tungsten mine in the Bergslagen district, Sweden. The conceptual framework for developing effective and efficient methods to recover critical metals and minerals from historical tailings has been divided into six steps: (i) Identification and exploration; (ii) Repository characterization; (iii) Tailings characterization; (iv) Metallurgical test work; (v) Process design and analysis; and (vi) Residue management.The historical tailings in the Smaltjärnen tailings repository were generated in the period 1935 to 1963, from primary ore of average grade 0.3-0.4 wt.% WO3, 0.2 wt.% Cu and 5-6 wt.% fluorite. The exploited minerals were scheelite for W, which was also the main mineral of interest in this thesis, chalcopyrite for Cu and fluorite. The repository is estimated to have about 2.2 million tons of tailings, and covers an area of 26 hectares.A site-specific sampling strategy and technique was identified, and based on the observed lithology, tailings particles were studied to understand their distribution across the repository. Methods for tailings characterization were identified involving drill core screening, particle size analysis, chemical analysis, X-ray diffraction, and mineral liberation analysis. Based on a comprehensive literature survey, an assessment of earlier processes from which the Yxsjöberg tailings were produced, and combined with the tailings characteristics, possible separation methods were pre-selected, including dry low-intensity magnetic separation and high intensity magnetic separation, enhanced gravity separation using a Knelson concentrator, and froth flotation. From the metallurgical test work, possible reprocessing flowsheets were then determined. In addition, an innovative mechano-chemical leaching process, referred to as leaching while grinding was tested.Historical tailings in the Smaltjärnen repository contain critical raw materials including W and CaF2 but also minerals of environmental concern as pyrrhotite and pyrite, in the dominating particle size fraction –600 to +149 μm. The average WO3 concentration in these tailings based on the sampled locations was 0.15 %, giving approximately 3300 tons of WO3. Scheelite recovery was enhanced using a Knelson concentrator, with the highest W recovery of 60.6 wt.% in the first rougher concentrate. With the maximum recovery of W attainable via the mechano-chemical method of LWG at 90%, approximately 2970 tons of WO3 would be recovered in the leachate for subsequent processing.