Reprocessing potential of pegmatite tailings for rare metal extraction and brick fabrication, Uis, Namibia

Historic tin mining and mineral processing of the Uis pegmatite deposit (Namibia) has resulted in a series of tailings impoundments. In this study, Uis tailings were characterised to establish their suitability for brick manufacturing and reprocessing potential for rare metals (Be, Li, Nb, Ta). Tailings occur on-site as two distinct types (a) fine- to medium-grained sand (d50 = 120 to 420 μm) in dune-like spoil dumps; and (b) fine- to medium-grained sand (Fraction A: d50 = 120 to 410 μm; ~25% of tailings type) and fine-grained silt (Fraction B: d50 = 18 to 83 μm; ~75% of tailings type) in four low-lying dried-out waste impoundment cells. The two tailings types have a similar and homogeneous mineralogical composition, comprising quartz, plagioclase, K-feldspar and mica as well as minor cassiterite, columbite-tantalite and petalite, reflecting the micaceous quartzo-feldspathic composition of the source rocks. The main clay mineral is illite with minor (ferroan) clinochlore, cookeite and kaolinite. Such a mineralogical composition allows contemporary brick fabrication due to illite as the main clay mineral, a large proportion of quartz as a stabilizer and only trace amounts of minerals that could have a detrimental effect (e.g. Fe oxides and pyrite). Consequently, the consistent particle size distribution and homogeneous mineralogical composition allow present-day production of high-quality fired bricks from tailings. To date, the use of Uis tailings for brick manufacturing supports the local economy, aids building development and removes silica-rich mine wastes prone to dust dispersion in a remote part of Namibia. In addition, geochemical analysis of the tailings reveals strongly elevated mean concentrations of rare metals (~2 760 mg/kg Li, ~140 mg/kg Be, ~41 mg/kg Ta, ~55 mg/kg Sn, ~74 mg/kg Nb). Resource estimates suggest the presence of ~25 200 t Li2O, ~1 600 t BeO and significant Nb-Ta-Sn in impoundment cell material and a total resource of ~365 000 t Li2O and ~27 900 t BeO in bulk tailings. Thus, Uis tailings also offer the opportunity for reprocessing and extraction of rare metals.

Assessment of Native and Endemic Chilean Plants for Removal of Cu, Mo and Pb from Mine Tailings

In Chile, 85% of tailings impoundments are inactive or abandoned and many of them do not have a program of treatment or afforestation. The phytoremediation of tailings with Oxalis gigantea, Cistanthe grandiflora, Puya berteroniana and Solidago chilensis have been tested in order to find plants with ornamental value and low water requirements, which enable reductions in molybdenum (Mo), copper (Cu) or lead (Pb) concentrations creating an environmentally friendly surrounding. Ex-situ phytoremediation experiments were carried out for seven months and Mo, Cu and Pb were measured at the beginning and at the end of the growth period. The capacity of these species to phyto-remedy was evaluated using the bioconcentration and translocation factors, along with assessing removal efficiency. Solidago chilensis showed the ability to phytoextract Mo while Puya berteroniana showed potential for Cu and Mo stabilization. The highest removal efficiencies were obtained for Mo, followed by Cu and Pb. The maximum values of removal efficiency for Mo, Cu and Pb were 28.7% with Solidago chilensis, 15.6% with Puya berteroniana and 8.8% with Cistanthe grandiflora, respectively. Therefore, the most noticeable results were obtained with Solidago chilensis for phytoextraction of Mo.

Physical and numerical modelling of drainage and consolidation of tailings near a vertical waste rock inclusion

The use of waste rock inclusions in tailings impoundments is a recent technique that offers many advantages, but it also raises a few technical issues that must be addressed to optimize their design. A laboratory physical modelling study was conducted to assess the effect of waste rock inclusion on the behavior of initially saturated tailings in terms of drainage and consolidation. The evolution of pore water pressures and settlements after hydraulic deposition of the fine-grained tailings (slurry), with and without a drainage inclusion, has been monitored and analyzed. This investigation also focused on the evolution of the tailings void ratio and volumetric water content, the amount of water transferred to the waste rock, and the movement of fine particles at the interface between the two materials. The experimental results are used to demonstrate how such waste rock inclusion can affect tailings consolidation by reducing pore water pressures with accelerated water drainage, for various imposed conditions. The experimental data are also analysed with numerical models to better understand the coupled processes involved. A discussion follows on practical implications of the use of waste rock inclusions in tailings impoundments.

Functional diversity of microorganisms in metal- and alkali-contaminated soils of Central and North-eastern Slovakia

A field-based study and laboratory tests were undertaken to determine the functional diversity of microorganisms in metal- and alkali-contaminated soils in Central and North-eastern Slovakia where iron ore and magnesite have been mined and processed for a long time. To improve the understanding of the functional diversity of microorganisms, we examined the effects of environmental factors on the functional diversity of microorganisms in metal- and alkali-contaminated soils in the emission field of heaps and tailings impoundments of iron ore mines (Central Spiš) and magnesite factories (Jelšava and Lubeník). Biolog^® Eco Plates were used to determine and assess metabolic profiles of microbial communities. The examined area of Central Spiš showed extremely high values of Hg and Cu and the values of Zn, Cd, Pb and Cr exceeding the permissible limit were determined. Soil reaction was extremely acidic to strongly acidic. The Jelšava and Lubeník area was characterized by alkaline contamination and the soil reaction was slightly acidic to strongly alkaline. The values of Cr, Mn, and Mg exceeding the permissible limit were measured there. The results indicate harmful and even toxic contamination. Our results showed that the diversity of microorganisms was different in the investigated areas and it was significantly influenced by environmental factors such as soil reaction, bulk density, porosity, and heavy metals Hg, Pb, Cr, Zn, Cu, Mn and Mg. Based on the results of the Shannon index, we can conclude that the diversity was low to moderate (2.5–3.3) and medium (3.3–4.0). Correlations between functional diversity of microorganisms and soil reaction, Hg, Cr, and Cu were determined. Our findings are decisive for understanding the microbial diversity in metal- and alkali-contaminated soils and they can be used to assess the quality and health of soil, as well as for scientific applications of remediation techniques.