Abstract
Rhenium is a very scarce element, occurring in the Earth's crust mainly carried by molybdenite (MoS2). Due to a very low availability comparative to actual industrial demand, rhenium is nowadays one of the most expensive mineral commodities and an increased interest is focused on ex- ploring residues resulting from a long-term mining, particularly of sulphide ore deposits. It is therefore noteworthy to assign the presence of rhenium (in a concentration up to 3 ppm) in the waste materials from the old sulphur factory at the abandoned mine of Sao Domingos (Iberian Pyrite Belt, Southeast Portugal), exploited since the Roman occupation of Iberia. Aiming at a potential sustainable recovery of rhenium as a by-product, X-ray near-edge absorption spectroscopy (XANES) was applied to clarify the Re-binding and mode of occurrence by comparing Re L3-edge XANES spectra obtained from mine waste samples (previously fully characterized by X-ray laboratory techniques) with similar spectra collected from Re-rich molybdenites (Mo1-xRexS2) and from Re-O model compounds configuring various valences and coordination environments of rhenium ions. Obtained results are commented, ruling out a possible Re-S binding and rather conforming with the binding of rhenium to oxygen in the analysed mine waste materials.
Highlights
The last naturally occurring chemical element to be discovered—rhenium—is highly valuable but very scarce, with an average concentration in the Earth’s crust estimated to be lower than 1 ppb [1].Rhenium occurs in Nature mainly carried by molybdenite, MoS2 [2] and its presence is assumed to be related with the volatile transport of other refractory metals—Mo and W—by high temperature magmatic fluids [3]
Studied Materials A fragment of a molybdenite museum specimen from Aldfield, Quebec/Canada—where this mineral occurs in a pyroxene-rock associated with pyrite and pyrrhotite [18]—was studied to collect the X-ray near-edge absorption spectroscopy (XANES) standard spectrum of rhenium hosted by molybdenite, Mo1−xRexS2—that is, Re ions with assumed formal valence 4+ and supposedly prismatic coordination by sulphide anions
Re L3-edge XANES spectra collected from the model compounds selected for the present study (Figure 4) match the spectral data published on rhenium oxide compounds, starting with works dated from the seventies [22] to the nineties [19] [20] [23], up to a recent publication [24]
Summary
The last naturally occurring chemical element to be discovered—rhenium—is highly valuable but very scarce, with an average concentration in the Earth’s crust estimated to be lower than 1 ppb [1]. An exploratory analysis of a wide range of elements performed on the sample profile of milled pyrite ore plus mining waste materials by an internationally certified laboratory [15] revealed that rhenium is present at concentrations significantly higher than its level at the earth’s crust (Table 2). According to these analytical data, possible correlation(s) with other elemental contents could be conjectured—e.g., with selenium. In view of the bulk chemical and mineralogical data, the sample displaying the highest Re-content (sample 4) was selected for a first X-ray absorption spectroscopy study, focused on assessing the binding state of this scarce metal in the debris from São Domingos old mine and desirably allowing enlightening the mode of occurrence of this valuable element in the original sulphide ore deposit for which the occurrence of molybdenite has not been reported
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