Rhenium occurrence in Portugal: An overview considering the valorisation of mineral resources

Rhenium (Re) was the last naturally available element to be discovered in 1925, occurring dispersed with a mean concentration in the earth’s crust lower than 1 µg kg^-1. This 5d element is positioned in the Periodic Table along the column of manganese (Mn, a 3d element) and technetium (Tc, a 4d element) but its overall behaviour is closer to molybdenum (Mo, a 4d element belonging to the preceding column). Molybdenite (MoS₂) is the mineral presenting the highest Re concentration. In some countries, Re has been extracted mainly as a byproduct of the copper extraction from sulphide Cu-Mo ores, once the metal is oxidized to the molecular compound dirhenium heptaoxide in the course of roasting of molybdenite, being subsequently recovered from the flue dusts under the form of ammonium perrhenate. In Portugal, the occurrence of molybdenite is mainly associated with quartz veins and in granite pegmatites in the north of the country, with Re contents varying between 0.1 to 0.9 mg kg^-1. Higher and unusual concentrations of Re (approximately 3 mg kg^-1), were identified in the abandoned São Domingos mine waste piles, located in the Iberian Pyrite Belt (IPB, SE of Portugal), related with pyrite ore roasting methods developed in the mine’s sulphur-producing plants, that operated between the 1930‘s and 60‘s. The binding state of Re hosted by molybdenite was studied by X-Ray Absorption Near-Edge Structure (XANES) spectroscopy using synchrotron radiation at the ESRF (European Synchrotron Radiation Facility, Grenoble/France) with the aim of understanding how Re is carried (hosted) by molybdenite - by diadochic replacement of Mo or forming Re-specific nanophase(s). Samples from the São Domingos mine wastes with high Re content were also studied, revealing a bonding situation to oxygen. In this chapter, and for the first time, an overview of the Re content in different geological environments is made. The most likely geological settings where Re may occur in appreciable quantities is investigated as well as where it is likely to be recovered; in ongoing mining projects, if appropriate minerallurgical investment is undertaken. Rhenium, obtained from molybdenite samples and for mine waste/ore samples both from Portugal is presented, considering the valorisation of mineral resources and the associated value chain.