Geometallurgical study of historical tailings from the Yxsjöberg tungsten mine in Sweden : Characterization and reprocessing options

Sammanfattning: 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.