Quartz, feldspars and REE from gneiss waste materials: an example from the VCO province (Piedmont, northern Italy)

Raw materials and critical raw materials (e.g. REE, PGM) supply is a matter of concern and a global challenge to face in a sustainable way: they can be exploited from ore deposits and be recovered from landfills (both urban and industrial), extractive waste facilities, and/or from waste streams. In a modern perspective of circular economy, the rational exploitation of mineral resources is essential, as well as a recovery and valorization of extractive and processing waste. In this research we present preliminary results related to the recovery of Beola and Serizzo extractive and processing waste, two commercial varieties of gneiss (dimension stone), quarried in Piedmont (northern Italy) and exported worldwide. Both Beola and Serizzo are varieties of orthogneiss, the former being very foliated and suitable for splitting, the latter more massive and used as granite for flooring and cladding. The percentage of extractive waste from quarry sites represents about the 60% of the total exploited material: it is possible to estimate an amount of about 110,880 t/y. Another important waste, whose management is very challenging, is represented by residual sludge (EWC 010413): sludge production is estimated in about 17,700 t/y. A critical point is represented by the regulatory framework of waste materials, with a view to their possible recovery. The relatively monotonic mineralogical composition of the gneisses consists of quartz, feldspars (oligoclase and microcline), and fair amounts of micas (muscovite and biotite). Typical accessory minerals are allanite (an epidote variety rich in REE), chlorite, and zircon. If from a mineralogical point of view there is substantial equality between Beola and Serizzo, the main differences lie in grain size and texture (finer grain and mylonitic microstructure for Beola). In the same mining district also granites are extracted, whose waste, after a series of treatments to remove ferromagnetic minerals (mainly biotite), are successfully reused in the ceramic sector (quartz – feldspars mix). For these reasons, also Beola and Serizzo could also have similar applications, if they undergo suitable mineral processing.  After extensive sampling and a robust mineralogical (OM and XRD), chemical (XRF and ICP-MS) and mineral chemistry (SEM-EDS and WDS) database, we have a complete picture of the characteristics of the waste materials from the different quarries. The main criticalities for the recovery of quartz and feldspars are represented by the grain size (especially for Beola varieties) and the relative abundance of phyllosilicates, which is higher than for granites. However, some varieties lend themselves much more than others to possible recovery, both for reasons of grain size and for the lower amount of mica. All varieties of gneiss contain fair amounts of allanite: this is a potential ore mineral for REE. One of the most easily processed waste materials for allanite extraction and concentration is sawing sludge: with relatively simple separation processes it would be possible to concentrate the heavy fraction, which also includes monazite (another important REE ore mineral). The efficient recovery of quarrying waste could therefore contribute to both the industrial minerals (quartz and feldspars) and the ore minerals sectors (REE).