1Luleå University of Technology, Luleå, Sweden
The development of green technology increases the cycling of several more “unusual” elements in society, e.g. Be and W. They are unusual in the sense that the geochemical knowledge of the
mobility, transport and environmental impact are limited. At the historical Yxsjö mine site (W–Cu–CaF2) in Sweden, the geochemical behaviour of Be and W were studied from both 1) tailings open to the atmosphere (Smaltjärnen Repository) and, 2) covered and water saturated tailings (Morkulltjärnen Repository). Several state-of-the-art findings were found by combining geochemistry with mineralogy within the Smaltjärnen tailings and by taken monthly water samples from the groundwater in the tailings and surface water downstream the tailings. Furthermore, re-mining was evaluated as a possible remediation method for the Smaltjärnen Repository.
In Smaltjärnen, pyrrhotite oxidation and too low calcite neutralization had decreased pH from 8 to 4 in the upper-parts of the tailings and formations of secondary gypsum [CaSO4] and hydrous ferric oxides (HFO) had occurred. Beryllium leached from the unusual mineral danalite [Be3(Fe4.4Mn0.95Zn0.4)(SiO4)3.2S1.4] due to oxidation and slightly acidic pH conditions. Released Be had
temporarily been scavenged by precipitation with secondary Al(OH)3 and CaSO4 within the tailings and at the shore of the tailings. In the groundwater, Be was detected in one of the highest
groundwater concentrations worldwide (average 4.5 mg/L). Beryllium released to the surface water had formed complexes with F- and was transported >5 km from the mine site. This is interesting since pH in the both the groundwater and the surface water was around 6, in which Be usually precipitates as insoluble Be(OH)2.
Tungsten has previously been considered as an immobile element. In the Smaltjärnen tailings, W had partly been mobilized from scheelite [CaWO4] by anion exchange with CO32- released from the calcite neutralization. Released W had adsorbed to HFO within the Smaltjärnen tailings and only low concentrations of W leached to the surface water. There it adsorbed on particulate Fe and settled to the sediments a few 100 meter from the tailings. Contradictory, high concentrations of dissolved W was found downstream the Morkulltjärnen tailings which were covered and water saturated. The concentrations of particulate Fe was low, and W was transported several km with the surface water.
A first step to evaluate the environmental impact of the surface water downstream the tailings was to study silican algaes growing on rocks. Preliminary results shows that the water quality had a negative impact on them compared to a reference stream.
These findings shows that remediation of the Smaltjärnen tailings is necessary. The release of contaminated neutral mine drainage will be ongoing for hundreds of years because only a minor part
of the tailings in Smaltjärnen have been weathered during the 50-100 years of storage. The results from Morkulltjärnen Repository showed that the traditional technique with cover and water saturation was not suitable for scheelite. Instead, Re-mining could be beneficial from both an economic and environmental perspective since Be and W mainly were found in their primary minerals.
Ph.D. student Hällström uses Environmental Mineralogy (mineralogy combined with geochemistry) to understand high-tech critical elements (Be, Bi, F, Ga, Ge, W) environmental behavior in skarn tailings and the downstream terrestrial environment, and evaluates the possibility to use re-minings as remediation for the tailings.