Tungsten (W) is listed among the European Union (EU) critical raw materials (CRMs) for its supply risk and economic importance. Primarily, tungsten is produced from scheelite and wolframite mineral ores with 0.08-1.5% tungsten trioxide (WO₃) grade. However, as primary deposits for these resources are becoming less or lower in grade, alternative sources need to be explored. These alternative tungsten sources include scrap from end-of-life products, mine waste and rejects from the ore beneficiation processes (tailings). The latter alternative source is the focus within this thesis.

Historical tailings repositories often pose environmental risks but may also become secondary sources of CRMs. This is because of relatively high minerals and metals content due to less efficient extraction methods and/or relatively low metal prices at the time of active mining. Therefore, reprocessing of such tailings is not only a supply risk-reducing measure but also an approach to remediation that contributes to the mining industry’s aim of moving towards a circular economy.

The aim of this thesis has been to develop efficient methods for separating valuable minerals from the tailings in order to leave behind a stable and environmentally safe residue. Geometallurgical studies were conducted by collecting drill core samples from the Smaltjärnen tailings repository in Yxsjöberg, Sweden, for evaluating the potential of this repository for further processing. The tailings were originally produced from the ore that was mined by Yxsjö Mines while it was in operation from 1935 to 1963, with average ore grades of 0.3-0.4 wt.% WO₃, 0.2 wt.% Cu and 5-6 wt.% fluorspar. The exploited minerals were scheelite for W, chalcopyrite for Cu and fluorspar. The tailings repository is estimated to have about 2.2 million tons of tailings covering an area of 26 hectares, with elemental concentrations of 1-2 wt.% S, 0.02-0.2 wt.% Cu, 0.02-0.3 wt.% W, 0.02-0.04 wt.% Sn and 0.02-0.03 wt.% Be.

Sampling and characterization of the historical tailings were conducted based on geometallurgical units (i.e. a distinction between different layers and locations in the repository), followed by metallurgical test work. The tailings were characterized with regard to color and granulometry, particle size distribution, chemical composition, scheelite mineral occurrence, texture and mineral liberation, as well as mineralogical composition. Based on a comprehensive literature survey, tailings characteristics, and assessment of the earlier processes from which the Yxsjöberg tailings were produced, feasible separation methods were pre-selected involving dry low-intensity magnetic separation (LIMS) and high intensity magnetic separation (HIMS), enhanced gravity separation (EGS) using a Knelson concentrator, and batch froth flotation.

The average WO₃ and Cu concentration in these tailings based on the sampled locations was 0.15 % and 0.11 % respectively. Applying them to the estimated 2.2 million tons of tailings in this repository gives approximately 3300 tons of WO₃ and 2512 tons of Cu. From the metallurgical test work, several feasible processing routes have been identified that need to be further assessed based on the economic and environmental criteria.

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.