RESUMEN

Scandium is a critical raw material that is essential for the EU economy because of its potential application in enabling technologies such as fuel cells and lightweight materials. As there is currently no secure supply of Sc, several projects worldwide evaluate potential Sc sources. While elsewhere in Europe emphasis is placed upon secondary resources such as red mud, we investigated the potential of industrial garnet sand and its waste products. Since Sc readily substitutes for Mg and Fe in the crystal lattice of garnet, the garnet minerals almandine and pyrope, in particular, may show high Sc concentrations. Garnet sand, after being used as an abrasive in the cutting and sandblasting industry, is recycled several times before it is finally considered waste which eventually must be disposed of. Extraction of Sc (and rare earth elements, REE) from such garnet sand may generate added value and thereby reduce disposal cost. The studied garnet sands from different mines in Australia, India and the U.S., and industrial garnet sands commercially available in Germany from different suppliers show average Sc concentrations of 93.7 mg/kg and 90.7 mg/kg, respectively, i.e. similar to red mud. Our data also show that "fresh" and recycled garnet sands yield similar Sc concentrations. Within the framework of a minimum-waste approach, it may be feasible to utilize the industrial waste-product "garnet sand" as an unconventional source of Sc and REE, that reduces disposal cost.

RESUMEN

This study reports column tests and modelling results to assess the impact of hardpans and cemented layers on oxygen supply in mine waste sediments. The analysed sediment samples were obtained from a low-sulphide and low-carbonate polymetallic mine waste tailings impoundment located in the Freiberg mining district in Germany. The three samples were characterised by different degrees and types of cementation. After physical and mineralogical properties of the samples had been determined, breakthrough curves of oxygen were measured in column studies at different degrees of water saturation, and the diffusivities were assessed using a numerical modelling approach. Results demonstrate that cemented layers and hardpans in undisturbed sediments associated with fine-grained material operate as preferential pathways for diffusive gas transport during rewetting, leading to higher oxygen diffusivities compared to disturbed sediments. Under air-dry conditions, the disturbed samples show higher diffusivities than the undisturbed sample, indicating clogging of the porosity by precipitation of secondary minerals such as trivalent Fe oxyhydroxides acting as a barrier and thereby decreasing the diffusivity of the undisturbed sample. In contrast to sediments without cementation, diffusion experiments of sediments with cemented layers used in this study yield similar tortuosities in spite of their different grain size distributions, pointing to the important role of these heterogeneities for gas diffusion.

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RESUMEN

This article reports fibre-optic oxygen measurements on a reactive mine waste heap located in the polymetallic sulphide mine district of Freiberg in south-eastern Germany. The heaped material consists of sulphide-bearing tailings from a processing plant of a lead-zinc mine. Mine waste material was deposited in the water phase after separation of mining ores in a flotation process. The tailing impoundment is partly covered with coarse sand and topsoil. Oxygen profiles were monitored during one year at eleven locations showing different physical and mineralogical compositions. At each location a borehole was drilled where the optic sensors were installed at 2-5 different depths. After installation the oxygen profiles were monitored seven times during one year from 2006-2007 and three to five oxygen profiles at each location were obtained. Oxygen measurements were accompanied by physical, chemical and mineralogical data of the tailing material. Additionally, a detailed mineralogical profile was analysed at a location representative for the central part of the heap, where the cemented layers show lateral continuity. Results showed that cemented layers have a significant influence on natural attenuation of the toxic As and Pb species owing to their capacity of water retention. The measured oxygen profiles are controlled by the zone of active pyrite weathering as well as by the higher water content in the cemented layers which reduces gaseous atmospheric oxygen supply. In contrast, gypsum bearing hardpans detected at three other locations have no detectable influence on oxygen profiles. Furthermore, the grain size distribution was proved to have a major effect on oxygen diffusivity due to its control on the water saturation. Temporal changes of the oxygen profiles were only observed at locations with coarse sediment material indicating also an important advective part of gas flux.

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