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

Abstract

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.